https://www.irs.gov/taxtopics/tc452.html
Where you enter the amount depends on if you received the support or paid it.
If you received alimony, go to the Federal Taxes > Income and Expenses
scroll to less common income and expand it and select the alimony received topic
If you paid alimony, go to the Federal Taxes > Deductions & Credits
scroll to Other Deductions and Credits and select the alimony paid topic
Hope this helps