There Is No Right Time
There is rarely a right time to come out with your feelings. Use your own judgment to discern whether your friend may reciprocate your feelings and when the best time would be to tell them. Consider that such a revelation cannot be taken back, and if the feelings are not reciprocated, you could potentially ruin your friendship. If things do work out, take them slow and embrace your new feelings. If things don't work out, it is important to treat the situation gently, no matter what side you are on, to preserve your friendship.
Don't Make It Weird
So you are finally enveloped in one another's arms after months or even years of pining over one another as friends, but now it's weird. Don't let the initial awkwardness overwhelm the beginning of your relationship. Embrace the initial puppy love that comes with the beginning of any new relationship. The subject of making it weird also applies to your group of friends. Your friends won't likely want to see your public displays of affection, especially during your group hangouts. They might feel threatened at first by all of the changes that are happening. Try to keep the showing of your affections private until your friends settle into your new groove.
Remember Your Friendship
Just because you've taken the step from friends to lovers doesn't mean you shouldn't still be friends. Continue doing things you used to do as friends: share secrets, play games and continue to go out with your other friends, either alone or as a couple. If you forget the people you were when you were friends, you may end up resenting the relationship and the subsequent loss of that friendship should things go sour.
Weigh Pros and Cons
Breaking up is always a possibility when in a relationship; however, when you begin as friends, there is potentially more to lose than there would be otherwise. Is dating really worth potentially losing your friendship? While a simple list won't suddenly halt your feelings, it would be wise to carefully weigh the pros and cons of dating your friend before you venture into a more serious relationship.