Welcome to Fluther.
Unless you are also a girl, there is no certain way to have unprotected genital-to-genital sex and ensure that no issue will ensue. In fact, if you’re potent and she is fertile, then there are even ways for her to become pregnant without penis-to-vagina contact. Life is pretty robust, after all.
Of course, that’s completely aside from the possibility of STD, which you have not mentioned, though other respondents have.
If you think you’re really ready for “the next level” in this relationship, then I would suggest that “the next level” involves learning more about the process of preventing pregnancy and STDs from certified experts. Talk to a doctor. In person. Don’t go looking for an easy out from random strangers on the internet.
If you think you’re ready to be a man “in that way”, then man up in this way first.