I am confused about 15A and 20A current.

Take the 15-amp circuit breaker out of your box and replace it with the same brand of breaker that's 20 amps. Email if you need directions for the removal and switch.

Unless your wiring is a minimun of 12 gauge (14 gauge is smaller) you cannot just change the breaker to a larger size. If you have 14 gauge wiring on that circuit try replacing the existing breaker with the same brand 15 amp breaker as breakers can wear out over time. If you still have a problem after the replacement, you're going to have to find another circuit with more reserve capacity or install a dedicated circuit.

You need to have a circuit that's capable of 20A as well as a breaker otherwise you'll end up defeating the point of having a breaker. While I'm pretty sure that standard 12/3 Romex is good for 20A, I'm not qualified to say for sure.

Do not just replace the breaker! In order to use a 20 Amp breaker you need a minimum of 12 gage wire. If the current breaker is 15 Amp then most likely the circuit is wired with 14 gage. I do not recommend you do anything to this circuit. To be honest if you don't understand the current rating you should not be messing around with the circuit. Not trying to be mean spirited, just trying to save your house and/or your life! As for the amp tripping the breaker, it if is the only item on the circuit there is something wrong with the amp. If there are other things on the circuit you should either use another exisiting circuit or have an electrician put another. preferably dedicated, circuit.

I never stopped to think about the possibility of an older home (etc) with old or small gauge wiring. One needs to be certain that the wire is at least 12-3. The posts above this post are correct and bring up some excellent points that I failed to mention. Another reason that I really like Audiogon :-)