Make sure it is one that allows you to see through it, so you can find what you want quickly. Avoid storing shoes in the original box they came in. You will forget what you have if you can't see it, and probably will be frantically running late and have to search for the pair you want to wear that day. The majority of closets have only one top shelf that goes entirely up to the ceiling. Add another shelf at the midpoint section and store less used items on your new top shelf.
Most closets have a shelf guide bar for the original shelf.
You will maximize your depth space available. Purchase shelf dividers for your usable shelf sections. Purchase drawer dividers for socks, underwear, lingerie and tiny items to keep items segregated. Consider reclaiming valuable drawer space and closet hanging space by folding jeans and stacking them on your newly acquired shelf space from the above suggestions.