There are two basic types of copyrights: PA and SR.
PA is what they refer to as "Work of Performing Art". This represents the actual song (melody, etc..). You want to get this for each song you want to protect. If you have a collection of songs by the same author, you can register them all with one form (I did this with my first CD since I wrote all the songs).
SR is a "Sound Recording" copyright. This is the copyright for actual audio recording. If the CD contains all originals you theoretically don't need this since the compositions would be protected by the PA copyright, but it's best to protect the recording as well. If there are cover songs, someone could take your recording of "Be Bop A Lula" and add it to their compilation CD once they get a Mechanical License (see below) for the song. Obviously not what you want.
My "Sing For Your Stew" CD contained original and cover music so I got PA copyrights for each of my songs on the CD and an SR copyright for the entire CD. The SR copyright can also protect the cover art as well.
It currently costs $35 per form (click here for current fees). On my project I had 6 originals and the SR for the whole thing so it cost $210 total (7 forms). They have a short form for the PA which doesn't hurt your brain to much but you do need to spend some time reading what some of the fields mean. Wikipedia has a great article on copyrights and the book "The Plain and Simple Guide to Music Publishing" by Randall D. Wixen is invalueable.
A Mechanical License is something you need if your CD includes music by other authors. If it's someone you know, it's not a big deal but if not, you need to aquire a Mechanical License for other people's work before you can sell it.
There are several ways to get a Mechanical License but one of the easiest (which I used) is through the Harry Fox web site: www.harryfox.com. They've got a nice form that walks you through the process. Fox is good for small quantities (50-1000 copies). If sales are better than that you can get better rates elsewhere but that hasn't be an issue for me yet. It cost me about $95 per song for a 1000 piece license (I can make 1000 copies of my CD). Their minimum is 500 pieces.
Also make sure to put "c 2005 your name" and "p 2005 your name" (circle around the c and p). The c one is for the cover art. The p one is for the sound recording.