I found this awesome article in the Family Handyman. https://www.familyhandyman.com/home-office/how-to-turn-a-closet-into-an-office/
There are lots of posts like that here on Hometalk, just search "closet office" and they should come up...like this one! https://www.hometalk.com/20904808/-closet-desk-makeover Hope that helps!
Ikea has a program on their website where you can design your cabinets. I would put in the dimensions of your closet and start playing. I would also look on Hometalk to closet/office reno projects for Pinterest too for ideas.