The best rule of thumb is if someone has transported something you own. You tip a waiter at a restaurant because they brought you your food, but you don't tip at McDonalds because you go up to the counter to get your own food, it's not brought to you. You tip a pizza delivery guy because he brought it to you, but you don't tip if you went and picked it up yourself. You tip a bellhop (or porter) at a hotel if they bring your luggage to your room, but you don't usually tip the doorman. You would tip a valet who parks your car for you. A restaurant is 10%-20% of the bill, a porter is $1-$2 per bag they have to carry. $2 is fine for a valet, $2 every time they bring your car to you. You don't have to tip when they take your car.

There are some places where people tip merely because there is a tip jar available... usually you just dump small change into there (the coins you get back or maybe a dollar). Starbucks comes to mind: there's no reason to tip at Starbucks other than the fact that there is a tip jar sitting out. If you're not used to tipping culture, ignore it. Same thing with ice cream shops or take-out windows. Absolutely no reason for there to be a tip jar there other than they're trying to guilt you into paying more than the advertised price.

There are certain situations where you might tip for extraordinary service rendered. You don't tip a doorman for opening a door, but you can tip if he calls a taxi for you. You don't have to tip at Starbucks, but if you ordered an extraordinarily complicated drink and they got the order right, a tip is called for. I usually tip even at take-out restaurants if I'm picking up an order late at night while they're getting ready to close the restaurant, or if it's a holiday and I feel bad that they're stuck working.

Some people get really mad if you don't tip a waiter. People rarely get mad if you forget to tip anyone else. So if you're confused, just tip your waiter 10% or 20% of the bill if they brought the food to you and forget any other tipping. If someone complains, just say you're European and aren't clear on the rules.