Alright, you're going to overcome this, so just relax, nothing wrong with taking it slow. First before pulling any moves be comfortable with her. Trust her. Care for her. Love her. Once you can say that you are/do all of the above, or even 3 of the 4 then it's time for fun. Start by hugging or cuddling, something light and nice and sweet, watch a movie, do something with her that you enjoy and will loosen you up. Realize why you care about her, why you find her so attractive, why right now, right then, you want to be as close to her as you can be. Now take her chin into your hand, tilt her head softly upward, and kiss her. Keep your lips tight for this, no sloppy kiss just a sweet sentimental one. Now take her somewhere private, that feels safe, like your room. Now make sure NO ONE WILL INTERFERE. Kiss her a little bit, cuddle with her, then say her name, when she says "what?" ask her "Will you go down on me?" this really is the politest best way i've found of phrasing it the first time. Ok now things are gonna get awkward if this is your first time, but just relax. Trust her. Trust yourself. Make sure she is comfortable (relatively) and you are comfortable. Make sure you keep contact with her, either hold a hand of hers, keep some OTHER part of your body close to her. Since it is your first time you may not come. Or you may very quickly, you get to find out. If you don't after 10 minutes, pull her up and kiss her. On the lips. No it isn't gross, she is the one you love. Make sure she knows that NOTHING she would do would make her "dirty" in your eyes.
You seem like a really nice person. I think things will be fine for you. Remember when two people are in love, nothing they do with one another is ever wrong, or dirty, or anything other than the closest you'll ever get to heaven.
Everyone starts somewhere, and you have the incredible luck to have your first experiences with her. I would kill to have my first experiences be with someone I loved. I would do anything for that. But I can't. You however can. Just remember this. Love. Commitment. Trust. Happiness. These are things you want from a relationship.
After you are done be sure to cuddle for around twenty minutes, maybe longer. Then you should spend as much of the rest of the day with her as you can.
Know this: It might not go perfectly, it might not be like you thought it would be. But it would be with her, the most important person on earth to you, so it will be wonderful because she is wonderful.
Best Wishes Bro.