First think about what you want to explore in the RP. Read the other RP's that are on here as well.
Always leave your partner something to respond to, be detailed but not overly so. I once had a partner who would write a dozen paragraphs a post but a lot of it was pointless detail, describing tablecloths and plates that kind of thing. Also they would just rehash a lot of what I had just written. When it starts feeling like work it becomes a bore.
Honestly, just jump in with a basic idea and play it out with someone. There are plenty of basic ideas in the request threads sections.