Details would be good, like what you plan on storing, if it's general household "stuff", I don't used full 2x4's any more, rather rip them in half, and screw them together, don't nail. Use the star or torx drive screws.
At the intersection of framing material, reinforce them with small gusset of plywood or metal it doesn't matter. Make sure you screw your plywood to the framing...it's a key part to making the unit strong, so it doesn't rack. If you have to, fasten it to the floor joist to keep it solid.
Don't put the stringers on the edge of he plywood, but move them in and you can support a lot more with less framing.
When you decide on a width of shelving, plan the size to minimize waste, for example if you use 2 foot wide, you can get two shelves from 4' material. If you choose 32" wide, you can use the two cut offs to make a shelf rather than having waste.