Common ones are on the pullstarter, motor mount, intake manifold, and gas tank. Check these screws carefully every week or so. Locktite can be used on them but will make removal difficult. When replacing screws make sure metric threads are used. You can find them at any hardware store.
Rear wheel is locked or clutch seems damaged.
Two problems can cause this. If the rear wheel turns freely for part of a turn and then stops it is likely that a 10mm wheel bolt has jammed your brake. Remove the wheel and check for this. If the rear wheel turns with great effort check to make sure both chains are loose. If you still have a problem it is likely your clutch is jammed.