1. By having unprotected sex with an infected person. Vaginal, Anal or Oral.
  2. By sharing needles, blades with infected person. (In the cases of drug abuse or even with by sharing simple shaving blade).
  3. By birth, from mother to baby, if the mother is infected. (Either by birth or through breast feeding, virus can be passed).

In short, HIV (and most viral infections) are spread through exchange of body fluids.