First off, congratulations to the both of you for finding each other, and being committed to each other. It sounds lovely.
As for coming out, there's no recipe, I'm afraid. You may want to make a list of people you're most afraid of coming out to and hold them off until later. Or, you may want to rip the band-aid off, as it were, and come out to them first. Other people prefer to be nonchalant about it and just, say, change their profile status to "in a relationship with <place name here>" You may want to do it together, so you can back each other up.
Like dartmouth said, you may lose friends. I didn't. Not even my fundamentalist christian friends or super-religious parents. Part of the scariness of coming out is not knowing what will happen. Remember that this can put a strain on your relationship. Whatever you do, do talk it out between the two of you. However you do it, make sure you're doing it for and with each other.
In this part of the world, in this day and age, it's pretty much a guarantee that some of your friends will be okay with your coming out - even if it comes as a shock (and you'd be surprised...it may not shock as many people as you think). When all is said and done, focus on them. Grieve the relationships you lose, but remember it's not your problem that homophobes can't get over that shit. Focus on the ones who stick with you. They are the people who love you for you, and not for who they think you are.
P.S. Good luck, and do tell us how it goes.
