We purchased our home at the end of July 2018. Do I enter the number on the property tax bill we received or do I need to prorate it?

[Texas Roger]

If you were reimbursed by the seller at closing for a portion of the 2018 tax bill and paid the entire bill in 2018, you must subtract the amount reimbursed by the seller from the total bill.

[tymustafa]

I feel like this is probably a stupid question, but... Where would I find that information? Closing documents?

[Zbucklyo]

Yes.  As xmasbaby says, you enter the amounts you paid, either at closing or subsquently (as Roger indicates, if you got a reimbursement from the seller at closing, subtract that; if you paid at closing add that).

[xmasbaby0]

You do not enter the amount on the bill--you enter the amount you actually PAID in 2018.