If you're going to suck your partner's dick (blowjob), put a condom on it first. Try non-lubricated or flavored condoms for this. Whatever you do, don't get semen (cum) in your mouth, because you could get an STD or HIV that way. If you have a sore throat or small cuts on your gums (say from brushing your teeth), there's a risk of HIV going from the semen into your blood stream.

If you're doing someone with a sex toy -vegetable, dildo, vibrator, or whatever put a condom on that thing! Don't switch the toy from butthole to vagina, or from one person's body to another, until you put a new condom on it.
Some people think that putting on a condom is too much work when you're supposed to be having fun. But it's easy for condoms to be really fun and erotic. Be creative!

You should know that at least 20% of people who find out they have AIDS are in their 20s. (OMFG)These people probably got HIV when they were in high school, because it can take years for an HIV+ person to develop AIDS.
HIV is transmitted from one person to another when certain body fluids are mixed-blood (including menstrual blood), semen, vaginal fluid, and breast milk.