If you have a friend or family member that comes out, try doing these things:

•Love them, remind them they are valid
•Respect them and listen carefully
•Do not out them or tell anyone about their sexuality (this includes family or whoever else). They will come out to whoever they want when they are ready
•Do not chalk it up as a phase or that 'they'll grow out of it'. They are telling you this because it is who they are, they have worked up the courage to tell you. PLEASE don't treat it as a casual thing (by that I mean, don't sweep it under the rug or 'forget' about it)
•Ask them about it to understand—if not, you can find many websites which are extremely helpful :D
•This is sort of like what I said before, but don't dismiss this as celibacy or being 'frigid' because it's not. This is what a person who is ace feels and you cannot change that.
•To all the aces out there, I know that coming out is scary and even if those you love don't accept you, just knownsomewhere there will be a place where people will accept and love you. Including here :)
•To all parents who don't know how to support or know what to say to their kids... just give them a tight hug. No words—just a hug :)
(Suggested by tiawolfandfriends )

I'm not an expert on this topic, but I still hope that these tips help you!
-Bat