Pull a chair up to the armoire and check the shelf heights. Armoires designed for a television will only have one shelf, and it will probably be at the right height to use as a desktop. Rather than trying to adjust the height of the desktop shelf, which may be structural, adjust the height of your chair. If there are additional shelves, measure the height of your monitor and remove shelves that are in the way.
Use a hole saw to cut a 1½- to 2-inch hole in the back of the armoire toward the bottom. When you set up your computer in the armoire, thread the cables through the hole before plugging them in.
Install a keyboard drawer under the existing shelf. Keyboard drawers slide out when they're in use and push back out of the way when they're not in use. Follow the manufacturer's instructions. Usually, you attach keyboard drawers with screws to the underside of a desktop.
Cut a piece of cork to fit the inside of one of the armoire's doors, or buy adhesive cork squares and trim them if necessary. Use a small paint roller to spread water-based contact adhesive evenly on the back of the cork and on the inside of the armoire door. Let the adhesive dry and then press the cork into place, taking care to get it lined up properly. Create a border by gluing ribbon or narrow strips of molding around the edges.
Attach wall-mounting file pockets to the inside of the other armoire door. Make sure they won't interfere with the contents of the cupboard when the door is closed, and use either heavy-duty double-sided tape or small screws to hang them so there are no screw ends protruding on the front of the door.
Install an under-counter light fixture on the inside top of the armoire, or the underside of the shelf above the desktop. Use your hole saw to cut a hole in the armoire near where the cord for the fixture attaches to the fixture, and drop the cord down the back of the armoire. Alternatively, use an inexpensive battery-operated light fixture.
Set up your computer in the armoire and determine where you have room for more storage. If you have the room and feel the need, use stacking baskets, small storage cubes or storage drawers.