Thick wire is coming from the battery and the thinner is going to the alternator. You should then have another wire which is the ignition switch signal to trigger the starter.

The 2 power cables will be on the 13mm nut and are red. The starter trigger wire should be black with a red trace.

To be honest, it sound like you have a dodgy solenoid on the starter and by tightening the nut up, you are pulling the contacts away from each other in the solenoid and they are not making contact. However, by leaving the nut loose, you risk it falling off and also the alternator not charging because it can't get a good supply of power form the battery and burning the stud off the starter as the current arcs across from the cable to the terminal stud.