Its been only a week since I purchased a Hyundai Sonata 2006 and the check engine already turned on what can that be?

Get the codes read at your local Friendly national auto parts store for free.

Check you gas cap. Easy to do and no harm, a loose or missing cap will light the CEL But I don't want you to think I mean that's the only possibility there are hundreds of reasons it will come on ---Do what BoholMark suggested

Definitely get the codes read. I've got 122K on mine and there were two codes when it came on about 10 months ago. First was an EVAP leak. Replace the gas cap - DO NOT BUY A UNIVERSAL CAP. You have to get the cap from the dealer. The second thing that was wrong was the power steering pressure sensor. I bought the part and fixed it myself. It's fairly easy to get to if you've got sockets the right size.