Accidentally over-tightening QR levers so you can't undo them again is a very common rookie error. Learn how to firmly secure them without getting them stuck.
Don't do up your QR as tight as you possible can before pushing the lever down - it will apply far too much tension, making it virtually impossible to release the lever again.
Make sure the wheel axle has located properly in the dropouts. Line up the QR lever so that when you close it, it will be parallel to the fork leg.
Wind the acorn nut on the other side until there is some tension, but not too tight.
If the tension is right, you will need to push the lever home firmly to tighten the wheel in the dropouts. If you need to apply more pressure than you comfortably can with the palm of your hand, it's too tight. Undo the acorn nut half a turn at a time until the tension is right.
Align your QR lever so it is flush with your fork leg or rear chainstay. This reduces the chance of it snagging anything and coming loose accidentally.