I went to my buddy the first time three weeks ago because my check engine light went on. It show error PO442 it said (Evaporative Emission System Leak Detected) sow we checked underneath the car and Didn't see any problems. So i must have left off my gas cap. Now this week the Light turned back on and I got the error code again Now what?

Well you start out with a leak test and check it very good for crimps and openings (under the car), to the hard lines that run from the fuel tank to the front fender where the evap canister is located. You could always try buying a new gas cap. If that doesn’t work try changing the “valve” that goes to the canister, you can’t miss it’s the rubber line thingy, lol... Also make sure your vac lines are hooked up properly to where you have no leaks or anything.

If all else fails get a new canister. I don’t have my vac line hooked up to mine and I don’t even get a code. It will not rob performance so don’t trip over it.

Live with it. You'll most likely never find the leak and never fix it.

So How do i turn it off the check engine light Completely just that warning?

I just explained how man, did you read the post above?. You can try erasing it from the ecu with a code reader but within the next few power cycles it will be back (if not within the first couple of miles).

Sooner or later you will learn that the CEL is your best friend so GET USE TO IT

EDIT:Well there is always the option of you taking out your cel light, that will get rid of the warning