- don't try to create too big a setting. It's usually better and time saving to create a kingdom, or even a single city, if it allows them to be fleshed out, and work from there outward, after a few months of campaigning. You'll get a feel of what needs to be developped then.
- The basics need to be covered. The geography/layout of the kingdom/city, what people can see in the sky, what normal people know about the kingdom's history, what is common knowledge, the recent news, the languages, and so on. Think from a character's perspective. What do they know after leaving 20 years in this area?
- Your background needs to involve some hooks for character development. If you and your PCs are into drama-RP, think about conflicts, contrasts that will give volume to your characters, NPCs and PCs alike.
There are loads of other things to be said on the subject, obviously. I suppose other posters will cover different areas.