Illinois contracts with Pearson VUE which provides for online registration and provides the insurance exams at multiple testing centers in Illinois. The biggest bit of advice we can you give is to try to schedule your exam several weeks in advance. (You are required to register at least 24 hours prior to your exam.) This does a couple of good things. First, it lets you put your study plan of action in place and provides an incentive to stay on track. Second, you want to make sure you have a spot when you are ready to take the exam.
If you need to change your date, that is normally not a problem, but you should do so with enough warning to avoid a penalty. The Licensing & Testing tab on this page provides you with succinct directions.
Go to the Pearson VUE website to schedule your exam.