Does circumcision really decrease the chances of getting HIV??

I don't think getting rid of the foreskin will make u less likely to getting HIV. However, circumcision does have its other advantages :D

HIV is passed through sexual contact, or getting contaminated blood mixed into your own (such as sharing needles, the unlikelihood of having an open wound and some of someone else's blood getting into it, or contaminated blood being transfused into your body through the accident of a surgical procedure, etc). Unless for some reason circumcision decreases the chances of someone getting into one of these situations, I am not really sure how it would benefit in the prevention of HIV or other STDs (or is it STIs now?).

It would not decrease the transfer of any stds as they are transmitted by most sexual contact. Most stds are also blood borne and can be transferred through microscopic cuts (i.e. semen from an infected male touching a micro tear in the vaginal canal of a female partner, or vaginal secretions touching a sore or cut on a penis, or blood of an infected person touching a cut on another person) it does not matter if you have foreskin or not, it holds no immunity to stds. That's why its always good to use condoms kids! They are not a full proof bet, but they are better than no protection, lol.