Thanks for your question and concern. Having sex for a woman with her male partner for the first time is a little uncomfortable because of the hymen as it has to break during the first sexual intercourse. But this might not be the case for all the women as for some it may have already broken as a result of the tampons usage or active participation in sports. And this might be the reason of pain in the vagina of your girl and not the STDs. It is extremely important for you and your female partner to discuss things and be properly aroused before you go for the sexual intercourse. You can probably enter your penis into the girl’s vagina in a very slow motion making your girl feel really comfortable and checking on if she still feels pain. You can also make use of various lubricants available in the market and make the penetration or intercourse really easy and painless to some extent. It is recommended that you should not use the lubricants that are oil based because there can be chances of the rubber to get rotten. You will definitely be able to have better sexual intercourse with the time. It is pretty natural for any girl to feel a little anxious or uncomfortable when having sex for the first time in her life. With practice, girls become comfortable, relaxed and confident. You can visit your local doctor or any nearest clinic of family planning along with your girl to understand more on having sex and learning about the precautions to be taken while having sex to avoid the STDs risks and unwanted pregnancy. Best Wishes.