I'm new to piano, I can play some stuff following the sheet music, but i'm always wondering if my fingers are on the right position, so, I'd like to know if there's like a map or a way to determine this? or people play however they feel comfortable with?
There are no hard and fast rules for this. See, everybody's got different sized fingers. There are plenty of ways to play the same thing. All you've got are just some general rules.
For the "piano lesson" book songs and SOME sheet music, there are little numbers next to the notes from 1 to 5.
They tell you which fingers to use on the note.
But this is (unfortunately) pretty rare. (Although extremely helpful) and sometimes given fingering won't work for YOUR fingers.
So you're going to have to work it out on your own most of the time. This is what you do in your first several "play through"s of a song:
Write it down (in pencil) on the sheet music !! (or in the midi sequencer - it lets you do that, right? :)
Use pencil, because you'll likely find a better way to finger it when you reach the next section or after thinkin' on it a bit (especially after a good night's sleep)
Watch the min and max range of notes for the section you're on, and figure out how you can make your fingers cover them best, with minimal hand position changes.
For chord after chord after chord type sections, you'll probably be moving hand positions. Otherwise, try to keep your hands somewhat stationary so you'll know what notes you have under your fingers easier.
If possible, try to keep your thumb or your pinky "anchoring" your hand.
Here's some more general tips from "Kreisler" of pianoworld.com's forums:
I have more info on my site at http://pianocheetah.com/practice/fingering.html