Compressor / Limiter help needed

[songpony]

Can anyone explain (in a simple manner) how to make this work? I have some songs that are really quiet, and I want to make them as loud as I can. I have other songs that are way too loud and I'm wanting to tone them down. With a few songs, the quiet parts are too quiet and the loud parts are too loud. It's my understanding that Compressor and/or Limiter changes do what I'm looking for, but I swear I've tried about every combination of things I can think of and I can't seem to make it sound any different. Any help would be greatly appreciated, especially if you can detail how the settings should be for each of the 3 specific scenarios I listed. THANKS!