register online before midnight here.
The November 4th election will decide an Illinois U.S. Senator, U.S. Representatives, state offices (governor, secretary of state, attorney general, etc.) and county offices (president of the Cook County Board, Sheriff, Controller, etc.) There are also some judges up for election, as well as Metropolitan Water Reclamation District Commissioners and several proposed Illinois Constitutional amendments and referendum questions.
City of Chicago elections (Mayor, aldermen, City Clerk, etc.) will take place in February.
Visit the Board of Elections FAQ for more information.