94 Ford Taurus wont crank only clicks Help!

I have a 94 ford taurus about 146000 miles. This car has had problems since I have had it practically, I have owned the car for 5 years. I put in a new radiator, replaced the started and the ignition. Replaced the battery cables, car has a severe corrosion problem with the battery cables what can I do to prevent it? Right now I go to crank the engine with the key and nothing happens, all I hear is a click. I had the battery tested and it tested good. I believe some cable might have got corroded? I dont even know where to look, or what to look for? Prior to the car not cranking, I would go out and I would try and start it at first it would crank really slow than finally catch. Until one fateful day it would not even slowly crank just nothing but a click. I would appreciate so much some instruction on what could be wrong and how to possibly correct it! OH And I dont know a lot about cars at all, so if you wouldnt mind speak in lingo a green horn will understand thanks again.