populate multi select from database, then save selected in database

[eluser]Unknown[/eluser]
Hello guys, i've tried looking but haven't found anything that could help me solve this problem.
I've created a simple video streaming site where videos can have multiple categories.

I wish to create video editing view with simple text input for title, link, etc, and a multiselect for adding categories to the video.
I want to populate multiselect from existing categories, and then save video with selected categories in the db.

Any suggestions?