I just started a book, but i cant decide if its better to use the present or the past tense (written in the first person)
It depends on what you are trying to do. Present tense creates tension because the reader doesn't know what is going to happen -- is the narrator going to live after the bomb explosion, for example? However, past tense can be advantageous if you want the reader to think that the narrator has learned from experience. Is the storyteller giving you the benefit of his/her wisdom after a long series of events? Is this a retrospective, or is it happening now?
A personal opinion only: I find it very difficult to write first-person present tense for anything longer than a very short story because anything I write seems rather false and 'pushed'. Of course, this is my limitation, but you might want to consider how many well-respected authors write this way before you take it on yourself.
You are almost certainly going to use both, since it is almost impossible to say very much without making reference to actions past, present, and future. But I think what you are really asking is, should you write in the narrative present or the narrative past.
The narrative present means that you are narrating events as they occur, as if you were walking down the street giving a running commentary on you life. "On the left I am passing the butcher shop. Now I am crossing the street."
The narrative past means that you are narrating events after they happened, the way you would normally narrate events, for instance if someone asked you how was your day?
Both the narrative present and the narrative past involve the use of multiple tenses:
Now I am passing the butcher shop where Dave met his wife. They are going to knock it down next month to build a Starbucks like they did last year on Main Street.
The narrative past is the default mode of storytelling in the western tradition (and probably in all the other traditions as well). The use of the narrative present is a device for obtaining certain effects. For instance, it can give the narrative breathless quality or suggests a sense of inevitability to events. Use it is your are trying to achieve such an effect and have some clear idea of how to do so.
There is no correct answer to this - the two tenses have different effects.
It is generally believed that the present tense is more immediate and gives a stronger sense of tension.
Whereas the past tense is more standard and tells the story in a more straightforward way. Personally I think you can build just as much tension in the past tense as in the present.
I would recommend writing the past tense if you're not that experienced, because it is the default standard for most fiction, and I believe it's best to focus on the story, rather than getting preoccupied by the format, when you're first writing a novel.
When you've written your first novel in a reasonably straightforward way, you will have learned an awful lot and you will be much better informed about whether you want to use one of the more unusual structures in your next novel, such as present tense. At that point you will be able to answer it for yourself, because you will know the impact it will have on your story.
One final note - because writing in past tense is so integrated into our psyche, you may find it harder to write in the present tense and will find that you often slip back into past tense without realising. Catching all these instances could be very tedious, but not catching them will make your writing look unprofessional. Just another reason to stick to past tense unless you have a damn good reason not to!
There is much ado about this question on the internet, with vehement opinions, but the simple truth is that readers don't much notice either way.
If you wonder about which tense you should use, the common sense answer is that you should use whichever tense you feel most comfortable with when you write. Try each for a day, and you will know.