As you are registered in Delaware as a C Corporation.Federal taxes - you will file form 1120.
State taxes - Delaware
In general, even though you are registered in Delaware, you are physically located in Illinois. Also, revenue is generated within Illinois. Therefore, most likely you will need to register in Illinois as a foreign corporation (foreign here as you are out of Delaware) and pay Illinois state taxes.
Sales taxes - This is an area you should pay attention to. Based on your type of business activity, industry -product or service, reseller or direct sale to the consumer there are various rules that apply in your situation.
If you have additional questions, I would highly recommend you consult a CPA or a tax professional so they can evaluate your sales & income tax compliances. The initial costs you will incur will save you lot of costs in penalties or interest if you do not manage this process.
** Based on a question asked on quora
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