As a rule of thumb, I will tip 10% on waiters or waitresses who provide average service, and up to 20% on servers who provide amazing service or when you have a large party.
You should calculate this number off the total value of the bill before tax. So if you had an amazing server, and your bill was $100 before tax; you would give them a $20 tip. If their service was just average, you would tip $10.
Hopefully this provides some insight into how to tip your server!