Here is my first attempt at building closet shelves. I am pretty happy with the way they turned out. Each shelf started out 2 feet deep. After they were in, I got on a step stool to see if I could reach all the way, especially the top two. I ended up cutting back the top two a few inches. I also had the kids reach into the lower shelves and 2 feet was fine for them.
Because I knew this closet would hold fairly light weight items, I used MDF strips that were preprimed. The shelving is melamine. It is cheaper to buy large sheets of melamine and have the store cut them down. Otherwise you can buy smaller, limited sizes. Plus, I wanted at least 2 feet for some of the shelves and the deepest precut were 18". This closet cost me $60.
I remember Jeff commenting that closets are a good way to practice your wood working skills. He's right. This was pretty simple, but I will work my way up to clothes closets.
Here is a link that might be useful: Closet project