You can not catch HIV from a mosquito bite, kissing, or sharing cups or hairbrushes with other people.  It is not passed on through normal contact. 

It can be transmitted through the sharing of needles amongst drug users, and even close sexual contact such as rubbing genitals together can pass it on.  If you think that there is a possibility that you may begin a sexual relationship, you should always carry condoms with you when you go out together, whether you are a girl or a boy. 

If you think that you may have an STD, you should see a doctor as soon as possible for treatment.