What is the best multiclass combination?
I am thinking in playing with a mage/cleric,is it good?
yes,i am a newbie

It is good.

In fact, all multiclasses are good.
However, since you're a newbie, I must suggest that you use ranger/cleric or any fighter multiclass.

mage/cleric, mage/thief and cleric/thief are not so good for newbies IMHO.

Ranger/cleric *is* a great class, it's what I recommend. Every cleric or druid spell in the book (quite literally) and you can use plate mail and any blunt weapon. It's excellent. For newbies, I recommend anything with high hitpoints and a good armor class -like a half-orc ANYTHING. Fighter, for example, that can wear all armor types and gets great hitpoints, or a half-orc barbarian - good AC, great hitpoints. The generic newbie class.

TRy using a shield; that helps when you're a newbie. Only a small point, but a good one when faced with the tempting offer of dual-wielding :p

If you've never played BG (I / II) before, then I'd suggest a pure fighter class. Just plain fighter, no kit (or at least take the berserker, there are no disadvantages). Should be fairly easy, even as a newbie. Change game difficulty to "Normal" (not "Core") and there you go!

Multiclass is more difficult because you won't get very high in either class (for example level 15 whilst 19 was possible) and it takes a damn looong time to hit an acceptable level (I, however, haven't played a MC yet, thus a bit biased). Experience points are divided, so every class gets half the current exp.

There is no reason not to take the mage/cleric even if you are a newbie if that is the class that you feel is desirable. Any class is going to be fine if you use the NPC's available to make a balanced party.

Melee tactics are easier to learn than magic tactics so the advice to take a fighter is not bad advice if you like turning your toon loose and letting the computer make most/all of the tactical decisions.

All of the classes are good enough to complete the game with ease if you do it with a party.

The only thing you should worry about since you are new to this game is balance. Have a bit of everything in your party. If you choose to be a mage/cleric just make sure you get some fighter type NPC's in your group. You don't have to have any class to complete the game (not even a thief, but since it's your first game I wouldn't recommend going without a thief, or for that matter a fighter, cleric, and a mage) Once you learn how things work and get more comfortable with the game you can create parties (anywhere from solo to six players) and not worry about taking any classes you don't want to. That is if you enjoy the game enough to play it again...and I think you will ;) For instance, I running through the game with 4 players, me a Dual Class Ranger/Cleric and Mazzy (fighter), Valygar (ranger), and Aerie (mc mage/cleric). I've really enjoyed this group.

I agree with pervious posts on choosing the mage-cleric for later... Mage-clerics have LOTS of spells (yup... you could get lost in their spellbook) and you level up slowly (which makes it harder for you to reach the level where you could now hold your own out). Try single classes or the basic multi-classes first. (fighter-thief, etc.) Then, when you've got acquainted to how the game works and how spells tick, you could get creative, aim for the more complicated classes with lots of casties and go as far as mastering the dual-class.

i think i will play with a kensai,and when i understand more about magic i will make him a mage.

SPOILER!!!!! In other words should he not take Airie (mage/cleric) with him because she is to hard to play? interesting..

NONSPOILER:
I think it is excellent to start with a cleric/mage if you feel like it. Its not like spells are rocketscience.

they can hide behind familiars really well too i hear!

lol hey cleric mages are nice. If you feel like you're in trouble just cast sanctuary and let your fighters pick the rubble

Nothing like a contigency filled with a sanctuary and heal/bless type spell to take a quick time out, heal, and get back in the picture. That's helped Aerie many o' times... :D

Basically, that's just the problem. The first time I played BG2:SoA I had no idea what spells do which, probably because I have never done a D&D game before. I didn't understand what AC was, THACO was, saving throws were and as for the spells, I had to spend LOTS of time reading what they could do, memorizing their icon and knowing their limits and capabilities. I had to watch in innocent amazement as Irenicus conjures his spells and realize in disappointment I couldn't do the same. I had to discover the memorization things by myself, what Lore was and why in the world my picture goes red when I get hit. Then there's the number of things here and there to remember....
Basically, the game's pretty much complicated, especially for one like me who never played a game like this before. (Before I played BG2, I was playing Heroes of Might and Magic 2 and Rage of Mages2) :)

I had to start from scratch too. But it's been great. *Laughs at how he once thought the mephit portal room was hard...and how he had to reload so many times when a yuan-ti mage in the Keep and a few trolls kept tearing him apart ;) :D

fighter/thief
fighter/thief
fighter/thief

=)