If two person were each others first time and never engaged in any kind of sexual activity before like kissing, oral sex, intercourse or genital contact, is it possible for them to catch stds?

If your concern is the commonly known STD's the answer is no. Two virgins who never acquired an STD can not give any to one another. However, any transfer of fluids, whether it by kissing, oral sex, or intercourse can transfer any germs the parties carry. Mono, hepatitis, flu, and many seasonal viruses can be passed through intimate contact so one could label them "sexually transmitted".