Learning to play the guitar?

I'm 15 and I really want to learn how to play the guitar. I've been in music classes pretty much throughout all my school years so i know what the notes and what the music symbols are and all that. My grandmother bought me a guitar when i was really little but i never got into it. But what i want to know is, is it hard learning to play the guitar and on average about how long will it take to get at least semi good to playing it?