If you flip the bike upside down (as you have done), then "pedal" in the "right" direction (as if you were trying to make the bike move forward), then let go of the pedal, the pedal should stay stationary (or maybe drift around slowly) and the chain should remain relatively straight and taut. If, when you let go, the pedal keeps moving pretty much at full speed or the chain bunches up, then the freehub/freewheel is not "free".
If that is the case, the first thing to check is for any sort of trash (leaves, string, etc) caught between the large rear sprocket and the hub. If you find nothing then the rear wheel needs to be serviced.
Since your mechanical ability is apparently quite limited, I'd suggest you take the bike to a shop, rather than attempt to fix it yourself.