The two most common ways to achieve landmark designation for an historic property or district is at the local level (through a municipality or county that has an historic preservation ordinance) or at the federal level (through the National Register of Historic Places program of the National Park Service). Landmark designation at the federal level is administered by the state historic preservation office; in Illinois, that agency is called the Illinois Historic Preservation Agency (IHPA). To find out if your local level of government has an historic preservation ordinance, contact the local department of planning office or see if it is a Certified Local Government (CLG), which is a local government with an ordinance that has been certified by the IHPA.
To inquire if your historic property is eligible for listing in the National Register of Historic Places, follow the preliminary evaluation process as instructed on the website of IHPA.