I want to put up a 6' x 10" shelf....width to be determined by structure that will hold up a 6' long shelf. The items on this shelf will be some books, craft supplies, knick knacks... Best way to approach this project?
If you can avoid floating, I suggest you do. It's inherently weak. (And sooner or later, someone's probably going to try chinups or an entire Encyclopedia Britannica on your work.)
If you gotta, Rockler sells these blind shelf supports: http://www.rockler.com/blind-shelf-supports. Hope you're in the US, or shipping/customs might be tough.
(My review on them is way at the bottom of the list, titled "somewhat disappointing".)
I built a torsion box sandwich of 1/2" baltic birch ply top and bottom with 3/4" x 3/4" oak between. Rockler suggests an extra long 7/16" drill bit for these, though you could avoid that if you drilled the back and middle 3/4" before assembly. (Post back if you want a drawing of the box construction... I don't know if I'm really describing it well.)
Only mount these to wood studs. One per stud would be optimum for strength. You'll want to turn them so the mounting holes are vertical, as the spacing for a 2x4 isn't quite right.
Thete are several kinds of supports sold specifically for this purpose, plus the option of using fairly standard triangular brackets upside down and arranging the shelf's contents to hide them.