I know that you have a lot of questions, and that's great! But we have an entire website full of answers to questions about sexuality, and lots of boards where people have already asked the questions that you have. So could you do us a favor and read the articles on the website first, and then search the boards before asking questions? We'd really appreciate it!
Here is a link to a whole section of articles that will answer your questions about STDs and STIs: Infection Section
Here are some links to articles about masturbation: Is Masturbation Okay? (Yep)
Going Blind - A Masturbation Memoir