You are caught in the age-old catch 22, no work to show in order to get work.
My advice: post on a forum and offer to cut some trailers for free. Find an independant producer who will gladly let you use footage in exchange for a free trailer. As they are not paying, you are likely to find people interested in this sort of thing.
This way you can cut a real trailer for a real film, and if you are good you can make a crappy film seem like it is much better (the true talent of a trailer editor).
I used to work in the trailer industry (doing graphics). While I find all the advice given here very good, I also know editors who got their first job by cutting trailers from real films. Just be very clear to a prospective employer that the trailer you are showing is not real.