I think the best thing to do is to film youself in a quickie video, then upload it to iMovie - immediately press all the buttons you can see, save and copy - everything. Play around with everything. I used to posess a mac with iMovie, and it was awesome.
Like in magic - don't try to expand on what you have until you've become proficient with what you posess. Same principle, really.
But if you must, check up on google - im sure there's lots of tutorials. You've just got to know what you want first, though.