Perhaps the most basic thing (and one of the most important things) to learn about video editing is how to split and trim video clips.
Splitting clips means that you break one clip into two or more parts. You would use this tool if you wanted to insert another clip between the two or if you wanted to edit out the middle portion of a video.
Trimming video is similar to cropping a photo, you cut off portions of the beginning or end (or both) of the video so that you're left with only the section you want.
To split the video clip, click and drag the bar that shows your position in the video. It's a black bar that goes all the way across your video timeline and when you drag it, the preview window updates with the still from that portion of the video. In Windows Live Movie Maker (WLMM) you can simply click on the "Split" tool on the "Video Tools: Edit" tab in the ribbon. If you're using Windows Movie Maker (WMM) click on the "Clip" menu and then click "Split."
Trimming the video is very similar. Drag the bar to the point where you want the video to start (or stop) and then click on the "Clip" menu again and select "Set Start Trim Point" (or "End"). If you're using WLMM, the buttons for the start and end trim points are right next to the "Split" button on the "Video Tools: Edit" tab.
If you don't like the splitting or trimming you've done, you can use the "Undo" option. Press "Ctrl+Z" on your keyboard to set things back the way they once were. Then try again until you get it just right.