Pick a topic that you can write about. It can be anythign: a short bio, a description of a job you do, a short story about something that happened to you. Then go to http://splasho.com/upgoer5/ and use the editor to write/re-write your text only using the top most common 1000 words. The editor will tell you when you violate the rules. 

The challenge (and editor) are based upon this xkcd comic.