Okay, I have a '78 Mobylette 50V that had the same problem.
Remove the spark plug (you may have to remove the long bolt that holds the engine in the mounts to lower the engine far enough)
Spray in some WD-40 and let it sit for a couple of hours.
Remove the chain guards (the white panels on each side of the bike, held in with three flat head screws)
On the other side of your engine (opposite the drive belt) there is a plastic bellhousing, it's held on with three rubber clips. pull the plastic bellhousing, and you'll have something better to use to turn the engine over. Turn towards the front of the bike only. You can wiggle front and back a little, but try not to turn the engine over backwards. If it won't turn, spray in more wd-40 and repeat the procedure.
You may want to clean out the fuel tank if the bike has sat for a while, and the carb may be gummed up. If you can get the engine unstuck, fuel it up (mix 3.6 -4 ounces of 2 cycle oil to 1 gallon of gas) turn the fuel valve to "run" , and turn the ignition switch to on. (same as the horn button)
Get on the bike and pedal like hell. If your decompressor valve works, pop that open while you pedal, and when you get going fast let it out, and it should pop over and start.
If at all possible, find a good motorcycle shop that can work on older stuff... After a few months of trying to get my bike to run well (it runs but not that well just yet) I've decided to drop mine off an have the pro's look at it.