I try to start the car, hear a click and lose all power?

Follow the smaller battery positive wire to see if it is bolted to the under-hood fuse box. If it is, check that connection to see if it's clean and tight. Follow the smaller negative battery wire to the body and be sure that connection is tight and not rusty.

If we need to look further, it will be helpful to have a helper to run the ignition switch while you take some voltage readings under the hood. You'll need a voltmeter or test light. Here's links to articles on how to use them, if you need them:

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester