Limiter or Compressor?

[djjstylz]

I have a couple questions. First, I am adding some samples and beats to an existing song I already mixed and burned to CD. Some samples and some beats are louder or lower than the original song. I want it all to end up at one equal level. Would i want to use a limiter or compressor or something else? Also, do i apply the limiter or compressor to the whole mix or the certain section i am worried about? Also, some songs i have already burned to cd sound like they need a little extra volume and or EQing. I am new to recording with protools so i don't know how to make it sound how i want it to. Is there a program or plugin that can do this for me once i am done recording and mixing with protools? I am working on a Dj mix so all my levels need to sound somewhat decent. Any help would be great. Thanks guys.

PT 6.4 Mbox
12" Powerbook 1.5
OSX 10.3.9