This is called masturbation. Some people don't masturbate at all, or hardly ever. Other people masturbate every day. Masturbation does not cause you any harm. Some girls enjoy touching their clitoris.
Many boys enjoy touching their penis, particularly the tip.
When a person is masturbating they become more and more sexually excited. This is when all the tension and excitement that has built up is suddenly released. Boys ejaculate when they have an orgasm and their penis will then go limp. For many people masturbation is their first sexual experience. Some people refer to masturbation as playing with yourself, or, especially with boys, jerking off, a hand job, or wanking.