First of all, please PLEASE make sure you have protection. If you're worried about getting caught up in the moment, one or both of you need to start carrying one with you. My first time hurt but only in the beginning, after a couple minutes it was nice.
By the third time having sex it was amazing! Once your hymen breaks, it doesn't grow back, so you don't have to worry about that. Also?
If he does things correctly, and gives you time to get excited and wet etc, there will be less pain. The more turned on you are the more room your body will create for him in preparation, and the more wet you will be! Good luck, have fun and be safe!
Almost forgot your question about position! That depends on you. To start with you may want to try something that doesn't allow him to go too deep, at least until you're ready.
Girl on top and from behind will allow him to go as deeply as possible inside of you, and until your comfortable I wouldn't suggest it! So, I'd try scissoring, spooning and missionary until you are ready to explore further!
SInce your hymne is broken already chances that you will bleed during your first time are very slim. You only will bleed if your boyfriend is really big or gets really rough but other than that you probably won't bleed. It probably also will not be painful unless you are really tense when he penetrates you, so don't worry about that part.
As for what position that's a tough one as this really depends on personal preference. It's great your boyfriend would like your first time to be special and romantic though sometimes things don't always go as you plan them so try not to create this idea in your mind on how it should be. That will only lead to disappointment.
Try to keep it open instead. Prepare yourself to experiment as much as you (and your boyfriend) feel comfortable so you can find out what you really like and/or turns you on. What is true for the position is also true for how long it will be.
I suggest you take your time but if things get really hot it can be over in no time. It's all up to you and your boyfriend. Hope this helps.