Hi and welcome to Just Answer!
The amount of child support is not determined on your tax return - but should be found in your divorce documents.
If you are looking to deduct child support on your tax return - please be aware that it is not deductible.
You may only deduct alimony payments to your ex-spouse - those are deducted on 1040, line 31 - http://www.irs.gov/pub/irs-pdf/f1040.pdf
If you deduct alimony - you need to provide the SSN of your ex-spouse and the IRS expects that amount to be reported on your ex-spouse's tax return.
Child support - however may not be deducted.
Let me know if you need any clarification of other help with tax related issues.
On a different note about calculating that child support using a 1040 form. Illinois courts base a non-custodial parent's child support obligation on that parent's net income. The courts are supposed to look only to the child support law, they're not supposed to go by what the IRS considers income. Net income is supposed to mean "all income from all sources" minus the deductions permitted by Illinois child support law.You cannot use the 1040 to really calculate the child support although you could look to the total income as a starting point.
I hope this information is helpful,