Tipping (给小费) is very hard in British and America. It's significant to gain this authority(权力)when you call at each place for it can decide whether waiters /waitresses, hairdressers or the taxi drivers may think that you are cute, impolite, strange, or whether they may run after you along the street!
In Britain
In Britain the reason for tipping is to show that you really like the service. In Britain the pay for any work is quite high. So when you tip, it is mainly showing politeness to the service person, and to show that they do a great job.
You can tip more or less depending on how good you think the service is, and usually the tip is around 10-20%. If you don't like the service, don't tip! Or if you are a student, or a stranger with not much money, it's also reasonable(合理的)not to tip.
In America
In America "tips" is often called "service". It is quite necessary to tip in America because the service persons in their industries have a low pay which they cannot live on, and therefore (因此) your tip is much needed by them.
It is in effect part of the payment that you make and is compulsory! Especially in restaurants, if you do not pay the service, you have not paid your whole bill, so watch out!