That check is a scam. Do NOT deposit it.
The game is this: You deposit the check and send a percentage back to the sender, thinking you get to keep the rest. However, the bank realizes after you have sent the money that the check is bad and charges you a fee. You've already sent the money, so you lose that, too. Of course, if the bad check caused your account to be overdrawn, the bank will ding you there, too. If you have already put it in the bank, call the bank immediately.
A list of real mystery shopping companies is linked at the bottom of this page. Sign up with them and you can shop. You won't get rich doing this, but it's honest work. There is also a "New Shoppers" area on this forum that will orient you to mystery shopping.
Hard work builds character and homework is good for your soul.