Speaking as a performer, I hate seeing audiences in shabby clothes. It is as if they are going to McDonalds or Park n Shop. The concert hall is designed to provide a dignified atmosphere, and the performer likewise treats the occassion with dignity, so please do not wear T恤 牛仔褲 波鞋. For men, at the very least put on a shirt with collar, proper pants and leather shoes. When you go to church or an occassion that deserves your respect, you don't wear T恤 牛仔褲 波鞋, do you? So treat the performer with equal respect.
Regarding applause: I assume that you are not a seasoned concert-goer. If you don't know the piece well, pay attention at how many movements there are and clap only at the end. It is a modern convention - in the olden days, you can clap after any movement or even request an encore of a specific movement, but in modern times, only clap after the last movement.
If the performance was really spectacular, a standing ovation is definitely called for - meaning, you can give your applauses standing up. This is the highest form of appreciation you can give the performer(s).
And please, please, please remember to switch off your phone.