Recently, I've tried writing down an outline of every chapter so when I start writing, I write the first sentence of every chapter and so have something to work on. At least if I then get stuck on one chapter, I can move onto the next and come back to the other chapter later.
When you sit down and actually write, think about what you're going to write first. Don't just jump in at the deep end before you can swim. Decide your setting, which characters you're going to use, what your characters are like and other such tidbits of information.
When you finally have this, start writing. And don't write in heavy description. Write simple sentences and then build up on them. I liken this to making a cake. Add the milk slowly. You can always add more milk but taking milk out of cake mixture is impossible, I think.
The beginning is always the hardest part for me but once I have the first chapter, I'm usually fine. So get your thinking caps on and think. It'll make writing all the more easier.