If your usual starting position is C3, then there you have it, you are writing in the key of C, just make sure that is your root note and you should be ok. Alternatively there are hundreds of guides for key scales around, just google it and you'll find hundreds.
its a little more complicated than what fletch mentions, but yeah, the note at which a song resolves is usually its key. here's a website with useful charts so you know what you're doing scale wise http://www.looknohands.com/chordhouse/piano/
start at ur root note then go in progression tone tone semitone tone tone tone semitone for the major scale, if ur going in semitones then its chromatic, lots more to it but thats some basic stuff for ya. if u go to ur local music shop u can buy music theroy books to work through which are very helpfull