Make sure the battery has a full charge and the charging system is ok? Use a digital multimeter across battery posts, everything turned off. The generic spec for a full charge is 12.6 volts. Also, make sure all battery cables are clean and tight, coming and going. If the battery is down, take it off and get it charged.
You can turn on headlamps, turn key to crank, watch the lites, if they go out or almost, usually the battery or cables, I can't rule out the starter at the moment.