After your business is legally formed, the business will need to be registered with governmental tax and revenue agencies.
The business will need to apply for and receive a Federal Employer Identification Number (FEIN) with the IRS. In addition, the Illinois Department of Revenue requires businesses to apply for and receive a state taxpayer identification number.
The above registrations will establish that the business is a taxpayer and determines what types of taxes that the business will need to pay (“tax liability”). A business' tax liability is determined by things like what type of entity it is (e.g., sole proprietor or LLC), the number of employees it has, whether it rents or owns physical office space, and other things.
The IRS has a lot of useful resources for small businesses to help them navigate and be prepared for the different tax filings that are associated with businesses. Those resources can be found at https://www.irs.gov/businesses
The Illinois Department of Revenue also has a page dedicated to businesses. The page can be found here: https://www2.illinois.gov/rev/businesses/Pages/default.aspx
I highly recommend printing out a hard copy of the final letter or confirmation pages sent by both the IRS and the Illinois Department of Revenue and saving a backup copy on your computer. I store two hardcopies of these records in a folder and have put a sticky note on each document that says “Master Copy” to make sure that I don't inadvertently throw it away or write on it.