HIV is spread by:

1. Having sex without a condom.  HIV infection can happen through anal, vaginal or oral sex if you don’t use a condom. Unprotected (condom-less) oral sex is not as risky as vaginal and anal, but still can spread HIV, especially when there are cuts, bleeding gums or canker sores in the mouth.
2. Sharing needles, syringes or drug works.  Sharing any of the equipment to inject drugs can spread HIV.
3. Pregnancy, childbirth or breastfeeding.  Without treatment, an HIV positive woman will transmit HIV to her child during pregnancy or childbirth about 25% of the time. Babies also become positive through breastfeeding.

Knowing how HIV transmission happens is the first step to preventing it.

Source:  http://www.aidshealth.org/learn-about-it.html/2/