How much you tip is up to you. Anywhere from 10% to 20% is typical. I do not go just based on a percentage. I usually tip based on the service I receive. If I get above and beyond service, I will tip high. If I get terrible service, like drinks going empty, having to ask for something over and over again, server not checking back with us, being very slow to bring the check, etc, I will not tip. I am one of those who do not believe a tip is an automatic given. If the service is acceptable or better, I tip. If the service is horrible, I won't.
Tip based on the level of service you receive. Some will tip $1-$2 per person at a buffet, since you serve yourself. $1 per bag for a bellhop. When our car broke down last year, we could only find a limo company to take us to WDW. We were about 60 miles away when our car left us on the side of the road. The rental companies were closed on Sunday. So they had a flat $20 or $25 tip they add to the cost of the rental. We told the company we'd tip him separate and did so. The driver was a blessing and we were so glad he came to get us and take us the rest of the way. He was great and friendly so we tipped $50. For me, its all in the type of service given. We live where the sales tax is high so we tend to just double it if average service is given.
You tip waiters, mousekeeping and bellhops. If you take a cab or car service from the airport, you tip the driver as well. Hope this helps!!