I might be coming out to my mother today, is this a good idea?

If your parents are cool with it, then it's absolutely a good idea. For one, it'll feel good to get it out in the open so it's just easier to talk about it from then on. Also, it's good to feel affirmed and supported by your family as well as your friends. I say if you feel ready and you think your parents will be good about it, go for it.