A good place to start, at least for the 2000 and 2010 censuses (and to some extent the American Community Survey estimates in between) is the Census site itself, American Factfinder. The new version of the site is WAY less user-friendly than the old site, a fact that their designers are aware of thanks in part to feedback from librarians and other regular users, but once you do get the hang of it - and if you have a good enough connection - there are a lot of useful features including collecting specified data by creating a selection from a map, and creating maps to display data.
Among the several tutorials available on the site is one on this specific subject. Finding a live tutorial offered by the census bureau through local affiliates, or someone who has attended such training, is another possibility.
There were a lot of sites out there that drew on census data - I'm not sure how many of them have proven cross-compatible since the factfinder redesign came out late last year. Hopefully Radical Reference volunteers will have some suggestions.
