1. Write stuff down in point form. This is helpful because usually the story comes out faster than you can write, but you could get down the rough points.
2. It doesn't matter where you start. Just go. Write from your point form and don't start from the beginning. Start from where it's easy. You can sort out how it connects up later.
3. Who cares if it sucks? You're not going to push out a prize winning novel on your first try. It's supposed to need revisions. So just get it out, accept that it will need more work and continue writing!