It depends what you mean by having sex. Having sex can sometimes mean a number of different sexual activities, but usually it means sexual intercourse. The most common definition of sexual intercourse is an act that involves a man putting his erect penis inside a woman's vagina. Sexual intercourse might also be used to refer to sex acts between two men or between two women.
Sexual intercourse between a man and a woman usually starts with them both getting sexually excited. This is sometimes referred to as foreplay, and might involve kissing and cuddling, touching each other and other sexual activities. Foreplay is important as it means a woman's vagina begins to get moist and a man gets an erection. If the woman's vagina does not get moist enough, then having sexual intercourse could be difficult or painful for her.
A variety of condoms Condoms can prevent pregnancy and the transmission of STDs
If a man and woman are having sexual intercourse, then using a contraceptive properly, every time, will prevent the woman becoming pregnant. If two people have sex and one of them has a sexually transmitted disease (STD) then they could pass it on to the other person. Using a condom is the best way to prevent any infection from being passed from one person to the other.
If a couple are going to use a condom for protection against pregnancy or infections, they should put it on the man's penis as soon as he gets an erection. Some men say they worry about using condoms in case they lose their erection or have difficulty putting the condom on. You could get some condoms and practice beforehand. Condoms come with instructions in words and pictures which show exactly how to use them.
After the condom is on, the man or woman can guide his penis into her vagina. The couple then move their bodies so that his penis moves up and down inside her vagina. This usually rubs the penis and makes the man sexually excited so that he has an orgasm. The movement might also rub the woman's clitoris (or sensitive areas inside her vagina) so she can have an orgasm. But this often takes practice and a bit of experimentation to get it right!