I put in a new alternator pulley an belt an new battery an my truck it worked for a couple of weeks now it bwon't start is it the starter?

New alternator pulley or new alternator and pulley? Punctuation is important

If you did not change out the alternator, check to see if the battery is being charged. That sounds about what a new battery would last if it is not being charged. Maybe the cables are not tight or need cleaning. Is the alternator hooked up correctly? Try a jump start, if it works, it is not the starter. Check the battery and see if it is good, and charged. Once I went through 4 batteries until I got one that didn't have a bad cell in it.