1. Don't Panic. Whatever you write now can be cut later so it doesn't matter.
2. Feel playful. This is just a little part of a journey, the first step. It doesn't matter.
3. Have an idea of your main character's problem.
4. You only need an idea as this will develop later, possibly in unforeseen directions.
5. No one can hold a complete novel in their heads, so don't panic if you don't know what's going to happen later on.
6. On the other hand, it helps to have a vague idea of where you're going. This is a story about a woman who has an affair that goes wrong. So, you need a scene with her meeting her lover for the first time. Then you need a scene showing her at home. Then a scene where the relationship develops etc.
7. When in doubt, have them get dressed to go out to an important event. Be specific about what they're wearing.
8. Another scene that normally turns up at some point is something about the place where they live. If stuck, write about that.
9. Write a list of their possible conflicts with other people - friends, family, the tax man... - then write a scene about one of those possible conflicts.
10. Alan Bennett said, "You only know what you're writing when you've written it." So get writing!