Taxis are easily identified, with their yellow or white colours, and they can be found all over the city. The meter price is subject to a supplement, which should be explained inside the cab, and consider that if the services are during the night or weekends, there is a further supplement added. The tip can go up to 10 percent, but beware of some drivers who overcharge foreigners! If you feel that this is happening to you, feel free to call a policeman.
Many cities are notorious for having taxi drivers that enjoy swindling tourists. Unfortunately, this is particularly true for Rome. It isn't difficult to find a taxi driver who is more than willing to take tourists from the airport to their accommodation for a previously agreed upon price - always higher, of course, than what the taxi metre would have read. Then there are those drivers who take tourists through the city centre, getting "lost" several times in the maze of little side streets. Given the heavy Roman traffic, the drivers do have an excuse, but you should still watch out for any obvious attempts to rip you off. In general, a trip through the centre of Rome should cost between 6-12€, depending on the distance covered.
If you still run into problems with a driver despite having taken all precautions, it is best to remain calm. All taxis have a number plate and the name of the company along with a reference number on the door. You can always jot down this information and file a complaint, which might not be resolved during your stay, but at least leaves a record of the occurrence.