Not very! Withdrawal is when a man takes his penis out of a woman's vagina (or "pulls out") before he ejaculates, or has an orgasm. This stops the sperm from going to the egg. "Pulling out" can be hard for a man to do. It takes a lot of self-control.
Even if you use withdrawal, sperm can be released before the man pulls out. When a man's penis first becomes erect, pre-ejaculate fluid may be on the tip of the penis. This fluid has sperm in it. So you could still get pregnant.
Withdrawal does not protect you from STIs or HIV.