" and installing a walk in shower "

Where?

We need more information. You say this closet is by your back door, does this mean it is in your kitchen or near the laundry room??? Why is the bottom part blocked off? Are there stairs under it? What did the part of the closet you're losing have in it? Do you need to be able to hang coats? Is the stuff that's in there now need to be there or could it go somewhere else? You need to know what you want to store there so you know if you need shelves or not, and if shelves, how many and how far apart to install them.

Sort, organize, and store items in separate baskets or nice boxes. Get rid of broken or never-used items.