[dirty hands]

first check the wire running from the positive terminal to the starter

make sure the wire is secure and corrosion free both ends

also look at the smaller ignition wire ..make sure it is tight to its push on

next check the alternator is charging

this may be a ignition key problem

if the starter turns over the solenoid must be ok

if it clicks or there are other noises from the starter

remove it and have it looked at or get a spare from a wrecker

if the car is an auto there is a vaccume device that fills with auto oil/or engine oil deposits and gives trouble you can replace that