When I started learning fiddle, I had a lot of tunes in my head already because I was a dance addict. I only had to figure out what key they were in/what note they started on, and pick them out.

I agree with everyone who's said to get the tune in your head first. You can use the software Jon mentioned to find the difficult notes.

I also agree with simple and slow to start off with. Simple because you'll be able to play the tunes and you can always add ornamentation later. A lot of tune books and ABC notation includes a lot of twiddly bits. You don't want the twiddly bits at first, and they're often just one person's way of ornamenting a tune. Find the "bones." Slow because you have to train your fingers to play the patterns. I find that if I make a mistake, I'll keep making the same mistake, and my fingers will learn it that way even if my brain knows better. Start slow and concentrate, then gradually bring it up to speed when you can play it without thinking about it.