How can I add different parts of iTunes songs into various clips?

Big Joon wrote:
... I tried using Garageband for this purpose, but it is sooo hard... I have opened a new file called "tutorial" for editing purposes. ..

Parts of your post seemed self-contradictory. However, if I understand you correctly, you wish to either add either multiple parts of one song or parts of three different songs to your project and the iTunes source of these songs are either audio music clips or "movie" clips. Further, you which each added segment to be faded in/out independently. If so, then you should be able to do this in either iMovie '08 or GarageBand.
Independent fading requirements means that each audio segment must itself be independent so as to have a user selected start and end point to which to apply a fading contour. In iMovie '08, if adding iTunes audio files to your project, simply pin them to a project clip and not to the background. Then use the leading and trailing edge of the audio icon to adjust your "in" and "out" point for the portion of the music you want played, as well as, adjusting the "fade in" and "fade out" contours as may be needed. Once satisfied with timing of your segments, merely slide them into their proper alignment relative to your video track.
If the source audio is on a video clip, it must first be imported to an iMovie '08 Event. Once in an iMovie '08 Event, you can use the Command-Shift key combination (i.e., hold down the Command and Shift keys while clicking on the Event clip) to extract the audio and drag the audio to your project. Once again, remember to pin it to a clip and not the background. Once added to the project proceed as described above to adjust the in/out points and fade contour and slide it to its final position.
GarageBand would work in a similar manner. If using iTimes audio music files, simply drag in your video clip and then add each of the audio clips as needed. In this case, however, you need to select the audio track you wish to edit before beginning work. Scrub the playhead to the in or out point you wish to create. Once located, simply use the split option to "cut" the audio track. Once the audio track has been divided into separate segments, simply delete those you don't want and use the "rubber band" audio control to add a couple of contour points and slide them up or down and/or left and right as needed to "contour" your fade however you wish.
If using audio from a video clip you must in turn add the video clip to your project and deselect the movie audio before adding/replacing the next video clip. In this way you will retain the previous audio track as each new clip is added. Once all of your music videos are have been added, you may then add you "project" video. Trim and adjust each of the music audio tracks as described above. I would normally do this as the last step in editing the project in order to avoid returning to iMovie '08 for any further editing. If you must perform editing in iMovie '08 as your final step, then you might consider the use Of garageBand as means or pre-editing the audio and simply add the edited audio to iMovie '08 for final alignment. I.e., you have any number of options in selecting a work flow that works best for whatever you are doing. Usually the hardest part is simply deciding which way you want to work.
ADDED: Oops! Looks like AppleMan explained things already.

Message was edited by: Jon Walker