In most states in America, tipping was there to make up the difference of the minimum wage and they pay a tax on their tips, so it's part of their income, not a bonus above their income so to speak. I know in Massachusetts the minimum wage was as little as $2.15 or something like that an hour!
You also tip based on how well you think the service was or wasn't. When tipping we used to work on the rule of thumb: 25% for stellar service, 20% for very good, 15% for ok and 10% for poor service.
Be careful though, with a foreign accent, as some may already have a 'service charge' already in the bill, but try it on for a tip as well.
You tip services like taxis, restaurants, valets, conciege/housekeeping, hairdressers/beauty therapists, Manicurists, Tour guides, masseurs, barman/women, baristas etc. Places like McDonalds etc may have a tip jar, but don't feel at all obligated to tip there.
It's a little tricky at first when it's not second nature to tip, but after a week living there, it will become second nature to you.