Well first of all, making a CD is quite complicated if you have no idea how to do so.
You're saying you downloaded instrumentals and want to make a CD out of it. Do you own the copyrights of those songs or did you ask it's owner if you could use it?
If not then don't, chances are you can get sued.
If yes then there are alot of programs.
Quote
*= free limited demo
**= requires a paid license
Completely Free software
* Anvil Studio -->Windows only
* Ardour -->Mac OSX / Linux only
* Audacity
* Kristal
* FASOFT n-Track Studio -->Windows only
* XO Wave -->Max OSX / Linux only
Mac & PC
* Ableton Live *
* Audacity
* Steinberg Cubase SX3 **
* Line 6 GuitarPort RiffTracker * (requires Guitar Port)
* Guitar Pro *
* Digidesign ProTools **
* Propellerhead Reason *
* Sonoma RiffWorks*
* Mackie Tracktion *
Mac exclusive
* Ardour
* MOTU Digital Performer **
* Apple Garage Band **
* Apple Logic **
* BIAS Peak Pro **
PC exclusive
* Sony Acid Pro
* Adobe Audition *
* Cakewalk Guitar Tracks Pro **
* Kristal
* Acoustica Mixcraft *
* FASOFT n-Track Studio
* MAGIX Music Maker *
* Pro Tracks
* Reaper *
* Cakewalk Sonar *
* Sony Sound Forge *
* Steinberg Wavelab **
* WavPad*
* FlexiMusic Wave Editor*
Linux Compatible -free software
* Ardour
* bplay
* Ecasound
* Gramofile
* KHdRecord
* Mix Magic -->Requires GNOME
* MusE
* SoX - Sound eXchange
* Traverso
* XO Wave
Anyways, if you have a mic, is it good? Does it records vocals correctly?
I would recommend you to try and buy a good mic, maybe even an Audio Interface that could help the recording. If you don't know anything about music recording/mixing, try to go in a studio and record some songs.
Other than that, if you just wanted to make a cd for fun, use your PC mic + audacity and good luck.
E/ If you want to make your own beats, try Fruity Loops. Although it costs some money, it's very very useful.