In Illinois I worked self employed for 4 months and earned around $5,500. By law is it required for me to file taxes since its such a low amount made? And if so how much would I have to pay, in estimate range for those taxesin 2008?

Yes, you MUST file tax return.
You must file your tax return if yourself employed income is $400 or more. You will report your income and expenses on schedule C or C-EZ (Form 1040). Then you put net income (or loss) on line 12 of Form 1040. This income is subject to SE tax at 15.3% (this is shown on line 57 of Form 1040). Half of SE tax is deductible on line 27 of Form 1040. Read: Your U.S. Tax Return: Tax Filing by Self Employed Sole Proprietor or Independent Contractor