Here are a few things to help you out:
- Have an idea
Even if it's as simple as "Character A meets Character B in a coffee shop". Have something. And expand from there.
- Have a character
If lieu of having a plot or even a basic premise, at least have one good, solid character in your head. Your main character. What kind of person is he or she? What is their goal? What do they want out of life? Who are they? Making a character profile can help.
- Just write
Okay, maybe you don't have any of the aforementioned. Just wing it. Don't think about it. Just write. Find a prompt online. Start with a basic place, or setting. "It was a dark and stormy night." See what develops from there.
- Don't edit
Keep writing. Even when you realize there's something crucial to your plot or backstory, don't go back to add it in. Make a note of it and keep writing. Get those fleeting thoughts out of your head. Don't stop to correct misspellings or typos either. There will be time for that later.
- Set goals
Give yourself a word count goal each day. It can be 500 words or 5000. It can be writing two paragraphs or writing two chapters. Whatever your goal is, stick to it. Write a little each day.
- Don't give up
This is the most important tip of all: just keep writing. When it seems like your novel really stinks and nothing fits together, don't stop. Just think of your favorite author. Their first draft of your favorite book probably sucked too.