Photoshop question

so, here’s what i’m trying to do:

i need to make a design for a t-shirt for the intro to guitar class at school that i T.A. for, and for part of it ineed a straight on sillouhette type pic of an acoustic guitar, like THIS

the problem:

whenever i try to extract it, it always comes out choppy. who dod i get it as smooth as possible? normaly i’d blur it, but that wouldn’t look good on a shirt…