Hey I was in a similar situation.
The girl was also one of my closest friends, if not THE closest friend I had in my friends circle. Last summer we've been hanging out a lot, and I started to get attracted to her. I was a virgin too, and hadn't had a real relationship before, whereas she had a lot more experience than me, sexual and relationship wise.
What I did, and I recommend you to do, is to go for it. Don't be afraid about that 'issue' of not having much experience. I sort of had this worry too in the beginning, but it faded quickly, because with her I felt really relaxed and not anxious at all. If you think too much and not let yourself enjoy being with her, then you'll do bad. As for kissing and sexual performance, it can be quickly learned and improved upon by a bit of practice

and reading some books on the topic.
You say you want this relationship to last. Well, don't put yourself under pressure.
If you realize that you and her don't really work out personality wise, be honest to yourself and her and end it. There is a lot to learn about somebody when taking it to the next level even if you've been close friends for a long time.
Be mature and a good boyfriend, but not a puppy. There is a lot of help to be found here in the forum.
Sadly in my case, the relationship suffers under bad circumstances. We are in an LDR. But no matter, I know for a fact that she never found any of my 'lack of experience' to be bad at all. It's all in your head, and self confidence will come to you quickly.
Good Luck,
~base_player