Trailers are always good presentations to introduce your book to a person. First you have to choose a cast. Who is going to play your main character?
For exemple, in "After" by imaginator1d, the author chose Indiana Evans to play Tessa so in all the trailers I see for that book, Indiana Evans is always there. You can just choose random clips with random people but it is easier if you choose an actor/actress.
You can also choose clips from a film. If you're writing about a Horror/Paranormal story, you can search for clips from Horror Movies. If you're writing an Adeventure story, then you have to search for Adventure movies, etc.
There are a lot of programms to do videos like Windows Movie Maker, Imovie or Sony Pro Vegas. I use Sony Pro Vegas a lot.
Personal advice:
In my opinion I hate when trailers have more than 3/4 minutes. It's called a book trailer and not 'the whole book in a 6-minute video' I just hate it when they're too long. There are 2 reasons why I hate videos with more than 4 minutes:
1-If the video is too long probably it will tell me the whole story.
2-If the video is too long, I'll be too lazy to watch.
Well, don't forget to choose a cast and do a SMALL presentation of your book!