You are just 21 years old and diagnosed with HIV/AIDS? That is really sad to hear man.
He will likely put you on steroids and assuming its your doctor for HIV/AIDS then he should be educated on dosing and so forth. To be on the safe side, do your research on Test and Deca and make sure you understand half times, typical dosages for your condition (should be plenty of studies containing HIV+ males, many of them possibly referenced in the AAS profiles on this site) and injection intervals so that you can make an educated argument if he tells you to take one shot every month and you know you need it at least every week.