Is this a DIY on a budget?
With no window, you might want to create a faux window o the short wall at the end of the room and make sure the room has plenty of light. You may want to add a heat/ac return register in the door for ventilation through the door and invest in a small fan to use this closet for an office but, in a pinch, it could be done.
Does the door open outward? If not, you'll want to check (from inside the room) how much actual room you have or change the door so it does open outward.
This one may not have the strength to hold the weight of a lot of stuff or someone leaning on it:
https://www.youtube.com/watch?v=-OvufFoUGBw
You could create a countertop with open space beneath it for leg room and storage beneath it.
http://jumptuck.com/2011/11/21/desk-build/
To use plywood rather than boards, you'll need to sand an already smoothed "finished" THICK piece of plywood to make it smooth enough to use for your desktop. If you don't have a circular saw, have Lowes cut the piece of plywood. You will need one piece for your countertop/desktop that is a right angle ("L") shape that is 18" deep.
You could cut the remaining rectangle in two pieces lengthwise and use them for shelves over the countertop.
To support the countertop, you could buy metal triangle brackets.
https://www.google.com/search?q=Metal+brackets+for+wall+hung+desk&tbm=isch&ved=2ahUKEwie3OvI7aXqAhVRFt8KHQXlDDAQ2-cCegQIABAA&oq=Metal+brackets+for+wall+hung+desk&gs_lcp=CgNpbWcQAzoFCAAQsQM6AggAOgQIABBDOgcIABCxAxBDOgYIABAFEB46BggAEAgQHjoECAAQGDoECAAQHlD70AVY7NAWYIrTFmgCcAB4AIAB3wGIAfEzkgEHNDAuMjUuMZgBAKABAaoBC2d3cy13aXotaW1nsAEA&sclient=img&ei=l0H5Xt60ItGs_AaFyrOAAw&bih=654&biw=1366&client=firefox-b-1-d
and/or create wood brackets:
https://www.pinterest.com/pin/86483255317012007/
and/or wood legs:
https://www.youtube.com/watch?v=dvH2KjP3SBo
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I like the color of the dresser in the background
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