Here's one way: 1"x4" or similar on the close back and sides, pine shelves, and then support in the center with either a) a heavy duty shelf bracket (into the stud) or b) install a vertical piece of pine (eg. 1x4) nailed or screwed right into the front of the shelf.

typo above: "closet"

Actually, if your shelves will be extra deep, then install a horizontal cross piece in the center, nailed front (into a vertical 1x4) and back (into the wall cleat). That way you can double up the shelves.

ok, so sounds like some sort of vertical peice is desirable in the middle. Actually, in my upstairs closet, I notices that is exaclt what they did. I could always run a 2x3 in the front from left to right too I guess as well....maybe even anothe set 14" back...maybe thats over kill and I would loose a bit of height in the aea the 2x3.

So are 30" shelves too deep? What material would I use...I guess I could get two from a 4x8 sheet...but sheet of what....particle board is so heavy...is there something else?

I'd build as a "U", 2 feet deep all around. That will leave a small step-in full height to stand a tall item. Make sure your doorway permits getting things on the upper shelves - they may need to be shallower.

Anchor a railing to the walls with a stiffener to the face of each shelf and vertical support at the inside corners. OSB is my choice. I wouldn't go more than 2 ft deep because it will be difficult to reach things that get pushed back, especially on the higher shelves.

U shape...interesting...let me look at that.....how thick should I go on the osb?

I'm sure 1/2" would work fine for storing boxes. I generally use 3/4 but can't say that it has been necessary.