When I go to start my gmc yukon it cranks but won't start, What could the problem be.

Lack of fuel or lack of spark. I would check the fuel pressure and see if there is spark at the plugs.

hello i'm a "back yard" mechanic, so here is the first thing you should start with is fuel pressure, if not try spark, and if not spark try oil level and transmission fluid level, also i should add that if its a manual shift try the neutral safety switch. here to help if you need me -RTBBilly

After I tried to start it and It wouldn't start I put spark plugs and wire in and I still won't start. No codes are coming up either. And when I unlock with the key the alarm goes off every time, I've only had the car 5 days,

Thank u all I'm a female and I'm trying to diagnose this myself I don't know much.

I just put gas in it and it started rite up.

I am glad it was easy.

try changing the air in the front tires. if that doesn't work, go ahead and change the air in the rear tires as well. if that doesn't do it, change the air in your head!!