Dual classing and Multi classing can be very powerful but for the individual new to the idea there is a lot to learn. I came across this page explaining it in great detail and it's very easy to read.
http://www.playithardcore.com/pihwiki/index.php?title=Baldur's_Gate:_Dual_and_Multiclassing
Here it is an excerpt from link, incoming wall of text:
"Basic Information
Dual and Multiclassing can be VERY powerful if used right in Baldur's Gate. Playing the game hardcore can be a lot easier if you duel or multiclass, as they have abilities of both classes with the only drawback being that you do not reach as high of levels. Also, if considering the entire Baldur's Gate saga as a whole, dual-classing allows for the most overpowered, broken, and downright cheap meta-gaming characters possible.
Dual-classing favors starting with your primary class as the combat oriented class, and then dropping into a spell-casting class. Casters scale better later into the game, and this way you maximize the early game health and thac0. Fighter should always be the first class for any potential fighter dual-class, and thief should be the primary class for thief/mages.
Fighter Multi-Class Options
Fighter makes the most universal dual- and multi-classing option because it offers something every other class wishes they had: HP and thac0. They hit harder and can absorb more punishment than any other given class meaning your squishy thief who couldn't ever land that backstab suddenly has twice the HP and rarely misses from the shadows, or your mage suddenly has more survivability than he would have enjoyed at max level just by dual-classing at level 8.
Fighter/Cleric
Very powerful solo class and great for hardcore runs, as sanctuary is basically a free 'get out of trouble' card. Using Draw Upon Holy Might can get your strength to as high as 23(!) at higher levels, making you the best fighter in the party, and boosting your constitution to 20 or more with the spell before sleeping will restore all your HP upon sleeping.
Strengths:
1. Divine Spells and therefore healing
2. Sanctuary allows easy safe navigation of the battlefield
3. Draw upon Holy Might turns your physical stats into sheer godliness.
4. Highest possible combined saving throws in the game (especially ludicrous when combined with a high con dwarf)
Weaknesses:
1. BG1 only: Will only reach 4th level spells by level cap
2. Fighting abilities progress slowly due to splitting experience, although this is mitigated somewhat by the ability to self-buff.
3. Limited weapon selection
What spells to memorize-
Because of the fighter levels and therefore higher thac0 than a straight cleric, focus on buffs when you chose your spells (Draw Upon Holy Might, Sanctuary needed, strength of one also good), taking healing spells only as necessary. Your first and foremost concern is keeping yourself alive, so leave debuffs and holding spells to your secondary divine spellcasters.
Dual or Multi?-
Multi-classing is preferred for a BG1 only character, as the early game survivability matters without sacrificing much potential end-game power. However for those planning on important the character through the entire saga, dual-classing to cleric at level 13 of fighter will maximize the attacks per round fighters get over typical clerics while still allowing you to reach level 38, a whole two levels shy of the default cleric level cap. Berserker is the only viable kit to dual-class into a cleric because heavy armors are a must in order to utilize the unique tanking ability fighter/clerics fulfill. Additionally, multi-class fighter/clerics are limited to mastery level in any given weapon type but a dual-classed fighter/cleric, even if he dual-classed at level 2, can obtain grand mastery in any given weapon type.
Choosing a Multi-class Race-
The only three races which can even roll Fighter/Clerics are Dwarves, Half-Elves and Half-Orcs. Half-Orcs are obviously only playable in BG1 if you install BGtutu or BGT. Half-Orcs fill this well due to the extra strength, constitution, and their ability score penalty applies in the preferred fighter/cleric dump stat. If that isn't possible or you feel it's cheating, Dwarves are excellent as the dexterity penalty isn't very severe, the extra constitution is extremely helpful and most importantly, dwarven saving throws make him very difficult to crowd control. There is no meta-gaming reason to roll a half-elf over either of these choices.
Recommended stats-
Max strength, dexterity and constitution. Get at least 16 points in Wisdom. Intelligence should be at least at a 9 to read scrolls, a vital thing for clerics. Charisma is the dump stat of choice.
Proficiencies-
Warhammers and Slings are musts. The best cleric weapon in Baldur's Gate 1 is a warhammer and is relatively easy to obtain early on, and slings are literally your only option for a ranged weapon until you get a returning hammer in BG2. Clearly sword and shield spec is a must as well, for those going into BG2 who can afford to split the proficiency points away from weapons.
http://www.playithardcore.com/pihwiki/index.php?title=Baldur's_Gate:_Dual_and_Multiclassing
Here it is an excerpt from link, incoming wall of text:
"Basic Information
Dual and Multiclassing can be VERY powerful if used right in Baldur's Gate. Playing the game hardcore can be a lot easier if you duel or multiclass, as they have abilities of both classes with the only drawback being that you do not reach as high of levels. Also, if considering the entire Baldur's Gate saga as a whole, dual-classing allows for the most overpowered, broken, and downright cheap meta-gaming characters possible.
Dual-classing favors starting with your primary class as the combat oriented class, and then dropping into a spell-casting class. Casters scale better later into the game, and this way you maximize the early game health and thac0. Fighter should always be the first class for any potential fighter dual-class, and thief should be the primary class for thief/mages.
Fighter Multi-Class Options
Fighter makes the most universal dual- and multi-classing option because it offers something every other class wishes they had: HP and thac0. They hit harder and can absorb more punishment than any other given class meaning your squishy thief who couldn't ever land that backstab suddenly has twice the HP and rarely misses from the shadows, or your mage suddenly has more survivability than he would have enjoyed at max level just by dual-classing at level 8.
Fighter/Cleric
Very powerful solo class and great for hardcore runs, as sanctuary is basically a free 'get out of trouble' card. Using Draw Upon Holy Might can get your strength to as high as 23(!) at higher levels, making you the best fighter in the party, and boosting your constitution to 20 or more with the spell before sleeping will restore all your HP upon sleeping.
Strengths:
1. Divine Spells and therefore healing
2. Sanctuary allows easy safe navigation of the battlefield
3. Draw upon Holy Might turns your physical stats into sheer godliness.
4. Highest possible combined saving throws in the game (especially ludicrous when combined with a high con dwarf)
Weaknesses:
1. BG1 only: Will only reach 4th level spells by level cap
2. Fighting abilities progress slowly due to splitting experience, although this is mitigated somewhat by the ability to self-buff.
3. Limited weapon selection
What spells to memorize-
Because of the fighter levels and therefore higher thac0 than a straight cleric, focus on buffs when you chose your spells (Draw Upon Holy Might, Sanctuary needed, strength of one also good), taking healing spells only as necessary. Your first and foremost concern is keeping yourself alive, so leave debuffs and holding spells to your secondary divine spellcasters.
Dual or Multi?-
Multi-classing is preferred for a BG1 only character, as the early game survivability matters without sacrificing much potential end-game power. However for those planning on important the character through the entire saga, dual-classing to cleric at level 13 of fighter will maximize the attacks per round fighters get over typical clerics while still allowing you to reach level 38, a whole two levels shy of the default cleric level cap. Berserker is the only viable kit to dual-class into a cleric because heavy armors are a must in order to utilize the unique tanking ability fighter/clerics fulfill. Additionally, multi-class fighter/clerics are limited to mastery level in any given weapon type but a dual-classed fighter/cleric, even if he dual-classed at level 2, can obtain grand mastery in any given weapon type.
Choosing a Multi-class Race-
The only three races which can even roll Fighter/Clerics are Dwarves, Half-Elves and Half-Orcs. Half-Orcs are obviously only playable in BG1 if you install BGtutu or BGT. Half-Orcs fill this well due to the extra strength, constitution, and their ability score penalty applies in the preferred fighter/cleric dump stat. If that isn't possible or you feel it's cheating, Dwarves are excellent as the dexterity penalty isn't very severe, the extra constitution is extremely helpful and most importantly, dwarven saving throws make him very difficult to crowd control. There is no meta-gaming reason to roll a half-elf over either of these choices.
Recommended stats-
Max strength, dexterity and constitution. Get at least 16 points in Wisdom. Intelligence should be at least at a 9 to read scrolls, a vital thing for clerics. Charisma is the dump stat of choice.
Proficiencies-
Warhammers and Slings are musts. The best cleric weapon in Baldur's Gate 1 is a warhammer and is relatively easy to obtain early on, and slings are literally your only option for a ranged weapon until you get a returning hammer in BG2. Clearly sword and shield spec is a must as well, for those going into BG2 who can afford to split the proficiency points away from weapons.