Ya gotta have compression, like HRD says, and fuel and a spark.
You can check for spark by removing a spark plug and holding the threaded part against a good ground while someone cranks the engine (I like to use insulated pliers for this as you can get a pretty good jolt otherwise). You can check for fuel by pulling the air cleaner and looking down into the carb while you goose the throttle. Gas should squirt into the venturi. You should be able to see and smell it. Doing these things should get you pointed in the right direction.
