Well....let's see here. It would help if you gave more info. Type of engine and drive train, for instance.
But let me assume that what you have is a 2 stroke w/chain drive to the left hand side of the rear wheel since that's the most common type.
Start off by pulling the clutch handle/lever in. (In other words, 'dis-engage the clutch') Can you now pedal the bike around like you would have before you installed a motor?
If the answer is yes, then prop the bike up some way that allows the rear wheel to spin. You might balance it somewhat over on it's side, resting on the kickstand. Or you might hang the bike from something above using a rope. Now let the clutch handle/lever out. (In other words, 'engage the clutch.) Then remove the spark plug. Now spin the rear wheel. Does it turn without much resistance. And can you hear the piston moving up and down in the cylinder?
The answers to these questions will give some good clues as to what's going on. So come on back once you've done these little experiments. Others, more knowledgeable than I, will have found this thread by then. You'll surely find your answer shortly after that.
Best of luck.