Http://www.uscis.gov/portal/site/uscis

All the information you need is here in the link above. You can organize e-filing, where your local offices are. All you need to know but beware it is long going and won't happen in a flash.

The USCIS site is definitely a good start, and USCIS has done a great job of improving it/making it more informative over the years - but doesn't have everything (it might have what you need, but it depends on what you're looking for.

USCIS is only one of the government agencies dealing with various aspects of immigration (it deals with immigration benefits - visas, green cards, work authorization, etc.). Relevant information can be found at http://www.ice.gov/ (ICE - handles domestic enforcement), http://www.cbp.gov (CBP - admission at borders/airports/etc), http://www.state.gov/ (Department of State - visas at U.S. embassies and consulates on other countries), http://www.dol.gov/ (Department of Labor - handles enforcement of employment authorization laws against employers and certain types of applications dealing with job and wage protections)

Each of these sites contains a lot of information about various aspects of immigration law and procedure, and even this isn't all of it.