As you already know by now that to shoot a movie on your iPhone 3GS, you just launch the Camera App, tap the mini slider below the still and video icons on the bottom right to switch from still to video, then tap the red button to start recording an iPhone movie and again to stop recording.
Because the iPhone's microphone is not very powerful, I would not rely on it too much to capture ambient sound.  Just focus on the visual action.
For the best video results, try to shoot in a room or outdoor setting with tons of natural sunlight.  The light sorce should be behind you, not i front of you - in other words, directed at your subject without casting them in a shadow.

To trim an existing iPhone movie be deleting frames from either the beginning or the end (those are the only two options Apple decided you only need) select the iPhone video clip in your Camera Roll and tap play.  To bring up the editing controls, tap anywhere on the screen.  You'll see a visual timeline of the frames that make up the clip.  If you shot your video in landscape mode - which is recommended if you plan on uploading your iPhone video to YouTube or edit it later on your PC - hold your iPhone horizontally as you edit so you can see more frames in the timeline at the top of the screen.
As you wath your iPhone video a progress bar shows where you are in the timeline.  This is helpful as a way to determine where you might want to trim the movies stating and stopping point at a certain spot, but it is far from exact.

With the timeline visible on the screen and your iPhone video playing, tap and hold one end of the timeline so the entire timeline has a yellow border all around it.  Without letting go, slide the end you chose to the right or left to lengthen or shorten the timeline, thus controlling the length of your iPhone movie clip.  To watch your selection, tap play.  If at any time you wish to lengthen or shorten the movie as you watch it, you can.  As you watch the iPhone video you are creating. the editing controls will disappear afer a moment or two.  Tap the screen to make them reappear.

After some trial and errors, you'll have a iPhone Video segment you're happy with.  To save it, simply tap trim to show the option to Trim Original iPhone Movie or Save Movie as New Clip.  To dump unwanted frames from the original raw video, choose Trim Original.  Otherwise, choose the econd option.  This saves the trimmed iPhone Movie as a new video in your Camera Roll.