First, check to make sure the battery terminals are clean, tight, and are in good condition. If they are and the ground wire to the engine block is clean and tight, make sure the starter isn't drawing too many amps. Take the vehicle to a nationally recognized parts store and have them check starter draw. Most will do it for free. The starter may be drawing too many amps. Usually, if the starter is the problem, the battery cables will get hot when trying to start.