Start by cleaning battery connections, baking soda and water, about a 1/4 cup in a big glass of water, let that sit maybe 5-10 minutes.Then rinse with water and don't get any on you, it'll have acid in it. Use a terminal cleaner, you can pick one of those up pretty cheap (take off neg., black first) and clean and re tighten connections, red, positive first. Maybe check the cable connections on the starter and engine block WHILE DISCONNECTED and check the connections on the solenoid (the battery cable will straight to it from the battery) on the fender under the hood, too. Good luck.
