A lot more info would be nice.
If there's popcorn on there now you do not skim coat it, you remove it all down to bare sheetrock.
Wet it and scrap it all off with a drywall knife.
Joe is right. If you try to skim a popcorn ceiling several bad things are likely to happen. Applying that wet mud will reactivate whatever keeps the popcorn in place and it will more than likely fall down. If you luck out and the popcorn basically stays in place, you will dislodge enough of the popcorn to mar your skim coating attempt.
Skim coating is a very labor intensive job. After removing the popcorn, you may want to consult with a drywaller or painter on the steps needed to procede. Also, there are literally dozens of threads on here covering this exact topic.