Once you foot your bill at a restaurant, the waiter will gladly take the money to the cash register, but he won't be getting a cent of it. Waiters in the United States depend on your tips to make a living. Think of that as they spend a few extra seconds describing or recommending you a special dish.
It's not that you need to do much thinking otherwise -- in the United States, tipping is such a wide-spread custom that it has evolved into an art with its own set of unofficial rules Here is a basic guide:
If you are passably satisfied with your waiter's service, your tip should be roughly 10% of total amount of the bill. If your order has been especially complicated or copious, you should up the tip to 15%. And if you are impressed with your waiter, either because he's been extremely helpful or extremely professional in the face of adversity, the minimum you're recommended to tip is 20%. 40-50% tips are not unheard of in exceptional circumstances.
Tipping, however, is not limited to waiters. Make sure to keep a couple of $1 bills in your wallet if you decide to go for a drink. You are expected to shell out at least one of those every time you're served a drink after you've paid for it. Just leave it on the bar wherever your drink was before you took it. (And do take it somewhere else. Lingering by the bar may be a natural thing to do in your home country, but American waiters aren't used to it and don't appreciate how it complicates their life.)
Remuneration is not all there is to tipping. Ever noticed how the words "gratitude" and "gratuity" are similar? No coincidence. Strange as it may sound to foreigners, you can thank people with money in America.
You won't be breaking any law if you don't tip a professional who goes out of his way to assist you, but it may seem as if you didn't acknowledge his extra effort. And efforts will be done for you as soon as you set foot in the country. For instance, a skycap may probably bring your luggage. He'll be used to receiving $1 per bag. If you or somebody you know is in a wheelchair and a flight attendant carries it, standards dictate a $3-$5 tip. Maybe you rent a car which is far away, and a man has to drive you in a shuttle. That man deserves $1. More if he helps you with the luggage.
Once you get to your hotel, a bellhop will likely take your luggage and carry it to your room, which usually begs for a $10 tip. If he opens the door for you and shows you your room, that's another $5 he's making. The next day, a chambermaid will clean your room. That should earn her $5 from you every night she does this. And you are riding a cab (as New Yorkers call a taxi), don't forget to pay 15% of what the driver asks you.
One night you may decide to give restaurants a rest and just buy some supplies from the supermarket (don't confuse this with a convenience store where you do everything yourself and no tip is necessary). If somebody at the register takes your groceries and starts putting them into a back, it's no reason no panic. That person is called a bagger and his help will you only cost you $1.