A newbie at this project, will try to document Hammarby Sjöstad as a starter. No other big plans right now.
Some tips, use JOSM as a offline editor and don't miss out the tutorial videos, a great way to get going. You need to have Java installed to use JOSM(its just a jar file), and if you are running Windows Vista make sure you install Java 6 or you will get into trouble.
Get software to record GPS-data at Making Tracks with Homebrew-ware, I personally use VisualGpsCE on a IPAQ and the use [GPSBabel] to convert the recorded NMEA log to GPX which is the format used by openstreetmap (Making GPX Tracks).