How can you practice for sex?

If you mean practice to have sex later, they can masturbate. If you are asking how to have sex, that is when a male's penis is placed in a female's vagina, and the male (usually) thrusts until he reaches orgasm. If you are asking how two males can have sex, then they would have oral or anal sex. Oral sex (among males) is when the penis goes into someone's mouth, and anal sex is when the penis goes into someone's anus.