I purchased something online and did not pay sales tax at the time of purchase. I need to pay it now. How do I report that on my taxes?

That is called Use Tax and is reported only on your state tax return, not the Federal one.

As you go through your state return you should see a question related to Use Tax.  When you see it, go through that section and report the purchase that you made.  It will usually be in the Credits and Taxes section of your state return.