go get this PA50 Owner's Manual and read the whole thing.. become familiar with everything on the bike..
Get this SHOP Manual
too .. it'll tell you how to fix things.
Spray a bit Starter Fluid into the spark plug hole.. reinstall the plug. If the bike starts up, the carburetor is dirty or no fuel is getting into it. If it doesn't start, chances are there's no spark.
The decompression valve in a little valve in the cylinder head.. Read the owner's manual.