Start by checking the connections at the battery see if you can twist either one . if you can move them they need to be tightened . Are they corroded if so clean them with water and baking soda. follow the ground wire down to the frame or motor and make sure you have a good clean connection there . Check the positive connection at the starter see that its tight . If you have access to a multi meter test the battery it should be 12.6+ . with the multi meter attached to the battery crank the motor If theres nothing and the voltage drops of abruptly it can be a bad ground or a dead cell in the battery. Start with this and let us know how it goes from there . Hope this helps