I want to build a shelf around a room to hold collector plates- how?

http://www.hometalk.com/diy/decorate/walls/diy-picture-ledge-15246467

Use quarter round, bought at Home Depot etc., they will cut to size. Paint it first, use glue on back to reduce the number of nails you have to use.

I like the quarter round idea. But, because that give a small space to put your plates on, I would also suggest placing a small piece of double sided tape, Velcro, or something from the 3M product line on the portion of the plate that leans onto the wall. You just never know, if someone bumps into the wall or something happens to jiggle the plates it just give extra security. Enjoy!