For more help with iMovie, go here.
Check the aspect ration. Choose 16:9 to match what YouTube uses.
If this is the first video you are making, create a new event and give it an appropriate name. For the next videos you make for the same project, choose "Add to existing event" and choose the event.
Drag videos from your event to the timeline. Drag photos from iPhoto or Photo Booth to your timeline. Drag and drop to rearrange.
Click the music notes to get the music menu. Drag and drop sounds or music to the timeline.
After inserting a sound or song, click the little settings icon at the upper left of the clip. Choose Clip Trimmer to change the length of the sound or song.
Click the transitions icon. Drag and drop transitions to the timeline.
Drag and drop titles to the timeline. Click on the video screen to edit the text.
The MOST important thing about your video is your STORY and content. You can put all the bells and whistles and transitions and titles and themes you want in your video, but if your story isn't strong, your video won't be good. Put your story first!!!!