I would get them out of your tank asap and take them right back to the store you got them from! If they are flatworms (or worse, Acropora Eating Flat Worms), they can be a pain to deal with. The stars are fixable with harlequin shrimp but if the LFS is selling corals with flatworms, I would find a new source for corals asap! If they are red planaria worms, you can likely get most of them with a dip in Revive or Lugol's. But I would advise you to just take the corals back and deal with two less headaches.....
+1 to Craig. I did not do anything special to get rid of the stars when I got them. They either died or were eaten. However, all flatworms are a pain and require some sort of chemical to treat. I try to avoid all such treatments because they can start chain reactions that are very bad.
If you can't take them back get them out of the display asap and get a dip to put in the container you put them in tomorrow.
Just an add on to my post above. It's not always the flatworms you can see, it's the eggs that you can't see that are an issue. Even if you think you get all of the worms off, there may be eggs that can infect the entire tank over the course of a few weeks.
Well to start off I would highly recommend buying some revive and start dipping EVERYTHING that goes into your tank! If you get into that routine it will help make everything so much easier for you in the long run. Just remember that it only takes one coral with one pest on it to contaminate your whole tank.
As for the stars, start grabbing them off the glass as soon as you can and get a harlequin shrimp to start eating them. The problem is that if ONE stays alive then they will be back in a month or two.
As for the flatworms. I've successfully gotten rid of them with flatworm exit. I don't know how big your tank is or anything but that would be the best bet to kill them. Other than that I would say get a sixline, corris, or leopard wrasse. Those will eat whatever flatworms they can find. Always nice to have one of those fish in the tank!
Are you sure the asterina stars are actually eating your coral? I don't have much first hand experience but from everything I have read they very rarely eat coral and if they are not eating corals considered to be beneficial to the system.
Maybe some others with more experience can chime in?