If you are getting the click when you turn the key then the solenoid is engaging but the hi amp contacts in the solenoid could be bad or, as previous answer, loose or corroded cables from battery to the starter.
Since it is cheap
, quick and easy I would first recommend checking all the cables from battery to starter for tight connection, corrosion
, etc. Don't forget to check both sides, + and - ground cables.
If that isn't the problem have you tried or do you know how to jump the starter directly from the battery? If that sounds like Greek to you then might be time to call a pro and spend some money