Hooking up with random people

Okay I know you guys have probably hooked up with different random people and i just wanna ask if aren't you afraid of having STD's or something? Like how will u know that guy you're gonna have sex with is clean? Like how do you guys avoid having STD's and AIDS when u actually slept with different guys? I've slept with only 2 guys in my entire life and I'm 19. I wanna explore more but I'm scared what might happen?