Replying to Dick Palais: Any edit to Wikipedia, no matter how small, is good even if it's just correcting a typo. One thing I've done is to make a small edit to every page I visit. There is always something to be fixed. Once you've done a number of small edits, you can do something more substantial. The mathematics reference desk is quite similar to MathOverflow and could be a good place to get started: http://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Mathematics.  

Replying to Marc Palm: It is quite easy to see who made edits to a page, just clicking on the history tab. A user's contributions can be seen by going to a user's page and selecting user contributions on the left.