Try to tip 10% at absolutely minimum, so a $19 bill would be around $21, but I would try to add $1 or $2.
If you pay with a credit card there will be a line where you can write down the tip, and sometimes they will have suggested tip amounts.
If the state you’re going to has 10% sales tax, you can just add however much the tax was, plus alpha.
Sometimes, at some establishments, if you forget the server will run after you and ask for a tip, assuming it was a mistake on your part. Not really that common though!
Hope this helped!