First, I think it is important for you to know that since you are both under 16 almost everywhere in the USA & Canada it is illegal for you to have sex.
Second, you need to know that no form of birth control is 100% perfect. So when you have sex you are ALWAYS risking the chance she will get pregnant. Before you have sex you need to talk about what you two will do if she does get pregnant - BEFORE you have sex the first time.
Third, since she is not on any form of birth control now - you have very few choices. Of the few that exist, the only one that does not require that she go to a doctor is condoms.
Fourth, if you are going to use condoms, you need to practice using them first.
As for position, that is a very personal thing. Different body types, different comfort in exposing ones genitals to others, different flexiblity, all contribute to what position is right for you.