You need a volt meter, common little cheap VOM, Radio Shack or any other. With the volt meter across the battery terminals, have someone turn the ignition key to the start position. If the battery voltage drops like a rock and the starter doesn't spin you probably have a bad battery. If the battery voltage stays constant or nearly so and the starter doesn't spin you have a bad starter or poor connection to the starter. Move the volt meter and read the voltage at the starter (from chassis ground) with the ignition in the crank position. If battery voltage is there the starter is bad, if not, again, you have a bad connection from the battery to the starter.