Hello, Yoga !
There are 2 games:
Baldur's Gate
Baldur's Gate 2: Shadows of Amn
And 2 expansions:
Tales of the Sword Coast for BG which adds a few additional sidequests and areas but nothing important to the story
Throne of Bhaal which adds additional quests and areas to BG2 and concludes the saga.
I don't recommend playing BG2 or reading the BG2 manual before playing BG because it totally spoils the BG story. The NPCs in the game will tell you what you've done in BG (even if you didn't).
About your character creation problem:
Here is a video where you can see how to create a multiclass character:
http://www.youtube.com/watch?v=Ex_3IwQ8RqQ
What exactly doesn't work for you ?
I disagree with twilight about the classes, clerics and bard are good choices. As cleric you get many hitpoints, can wear the heaviest armor and get many useful spells per day (thanks to the wis bonus).
Bards are also good in BG, they can identify everything, level faster and get more hitpoints than mages, cast mage spells at high casting level, have an useful bard song and can use bows.
A fighter fits perfectly to the other party members you want to have. But I wouldn't plan that far, just take whomever you meet and like.
If you create a new character make sure to get a good roll. The bonuses you get for maxing str, dex or con can help a lot. If you create an half-elf fighter/mage I recommend raising str, dex, con and int to the max, dumping wisdom to the minimum and putting the rest into cha.
For a multiclass mage I'd take a fighter/illusionist instead, you get better rolls at creation and the extra spell per level does make a difference.