Involve the girl for whom you are decorating a desk in the creative process. Ask her to cut pictures from magazines and search online for rooms and styles that appeal to her. Her favorite television shows and characters could provide inspiration in their set design.
Make a list of her decorative likes and dislikes, such as colors and patterns, and include her storage and desktop needs.
Plan the desk area by measuring the available space -- be sure electrical outlets are available -- and sketch the layout. Take photos of the room where the desk will be placed, and use colored pencils to experiment with color schemes and patterns she likes that will also complement the rest of the room.
Search inventory you already have for a desk or other furniture piece, such as a table, that can serve as her desk. If the existing desk color doesn't match her color scheme, paint it in one of those colors; drawers and/or fixtures can be painted a different color within the scheme for a decorative touch.
Shop for a desk or table if you do not already have one. Keep in mind the measurements of available space, storage needs and color scheme. If possible, buy one in a color that will work without changing anything, or buy one unfinished and paint it entirely one color or multiple ones within her color scheme.
Buy or cut plexiglass to cover her desktop. Place colored or patterned fabric that complements her color scheme underneath, or leave bare. Either way, the plexiglass will protect her desktop and give her a space under which to place favorite photos, cards and to-do lists.
Add desktop accessories, such as a lamp, pen holder, mouse pad and storage. Repurpose items she already knows and loves if possible. A glass, vase or candle holder can be used to store pens, and small bowls can be used for tacks and paperclips; household items generally are available in a wider array of colors, textures and patterns than storage bought in an office supply store.
Hang or place a magazine rack and shelving beside her desk to display current magazines and books she often uses. This will free up her desktop space and keep reading material in view and -- hopefully -- off the floor.
Buy or create a bulletin board for over her desk. Cork board and foam core are readily available in office and craft supply stores; cover either of them with fabric, pull tightly over the front and staple to the back.