The other answer is correct, but I wanted to give a more general answer that applies to *all* trading card games. Find a local store that specializes in selling the cards and, even better, hosts tournaments. The people running the shop:

 - Know exactly what you need.
 - Will teach you how to play.*
 - Will occasionally give new players free starter packs or cards (mine does).
 - Make far more money off of return customers than they ever will off of starter kits. They want you to come back, and so they generally won't try to screw you.

The point is that they want you to get into the game, and you want their instructions, guidance, and merchandise. Don't go to Walmart and leave wondering whether you bought the right thing for your kid.

<sub>*Unless a tournament is running (and if they *are* busy they will usually tell you when to come back and enjoy a quiet game)</sub>