I haven't really used MovieMaker a lot, although my wife has played with it a fair bit... lemme pull it up here and take a look.
First thing you want to do is Import your file using "Import video" under the "Capture Video" Movie Task (assuming the Movie Tasks pane is open - I believe it is by default)... if there are any "breaks" in the video, WMM should automatically create separate files, assuming the breaks are "chapters".
If WMM has started up showing the "Storyboard" view at the bottom, click the Show Timeline button - you'll get a timeline with "Video", "Audio/Music" and "Title Overlay" strips (if you already have this view and a "Show Storyboard" button, then leave it as is).
Now drag your video clip into the timeline - it should show the full length. If you want to shorten it, you can click and drag the start or end line of the clip. If WMM has split it up into multiple clips, you'll need to drag each of them onto the timeline in order. (I believe, BTW, that this behaviour can be disabled in the options, but this method will work fine for now).
Assuming, again, that the "Movie Tasks" pane on the left is open, once your clip(s) is/are all on the timeline, click on Finish Movie, and under that, select "Save to my computer". Give it a name and location and click Next... it should then give you an option for "Best quality for playback..." and a "Show more choices" link - click that link. Select "Best fit to file size" and set it to... say, 90MB, just to allow some leeway. Click Next, and away it goes - it will create a new file in WMV format that you should then be able to upload to YouTube.