Gerry is correct but there is an easier "go/no-go" way to see if you are getting enough power to the small stud on the starter mounted solenoid.
Assuming you do not currently have a starter relay or remote starter "solenoid":
If you get the dreaded "click-click" from the solenoid, and you are sure the battery is well charged and all the connections/cables are good, buy a remote starter switch (basically a long piece of wire with clip-on ends and a push button in the middle). Connect one end to the battery positive post and the other to the small post on the starter solenoid. If the starter then cranks when you push the button in the remote starter switch, it means that you are not getting enough power to the small terminal on the starter via your starter/key switch.
If that is the case, use the wire from your starter/key switch to trigger a relay. The relay can be a common 30 or 40 amp Bosch type but if you don't know how to wire one you can use a Mopar type starter relay (ask for a starter relay for a 1970 Challenger for example). The RobbMc starter instructions show how to wire it: