You should get a bill in the mail. Ask your county tax collector. You can probably find out online. I can look up my house in California.
Contact your county's property appraiser or tax assessor's office (or visit their website).
You should be receiving an invoice or tax bill in the mail and there is likely a method whereby you can pay online.
I will just add to the above that your assessor may already know that you paid off your mortgage, because the lender should have contacted them. But you should also do it yourself. You want to make sure the assessor has the right mailing address for you, and that you know what dates to expect your bill to be mailed to you and payments to be due.
I even get an email when the tax bills are going out. See if you can sign up on the county website.