In Illinois at age 18 you are likely to be in the "full licensing phase" of the graduated license process. With this being the case, it appears your penalties for driving with expired insurance would be the same as a driver that is fully licensed.

Illinois randomly checks vehicles for insurance by sending out questionnaires asking for the name of your insurance company and policy number. Your answers are then verified, if you do not have insurance your license plates will be suspended. If you are stopped for a traffic violation or in an accident, the police may cite you for not having insurance. If convicted you license plates will be suspended and you will face fines of a minimum of $400 for driving uninsured.

Your license plates can be reinstated with a $100 reinstatement fee and current evidence of insurance. If you are found to be a repeat offender you can be required to serve a four-month suspension period. Before being penalized by your state get a free car insurance quote here.