Can the joists support the weight of furniture and people? Is there easy access, or do you need to install stairs or a spiral? Does it have dormer windows, or are the walls something like this:
So assuming you answered yes to all of the above, here's how it works: Paint it a bold color you love. Use light colored furniture and fabrics, and paint the whole a bright, bold color you love. (You can also use wallpaper, but paint is easier for decorating novices.) For reference, reds are the hardest to paint, the hardest to maintain and the first to fade. Blues and greens are calming, but may be too calming for an office space. Yellows, oranges and yellow based greens (sage, lime, celery) are more invigorating, but may make you look sallow. White is actually hard on the eyes (glare, and we don't associate it with anything natural), and cream looks dirty in some lights. However, use a light color for the trim. Color is critical, as is placement of furniture.
Trim back any plants (ivy, trees, wisteria) that interfere with natural light, and add track lighting sources - halogen and LED bulbs shed a really clean, white light. This is not the place for incandescent lighting or table lighting because the former is more yellow and the latter is too low. You can make good use of the angled ceilings by installing lights on them. If you don't have plaster on the walls, this is not at all hard to do (running the lines are the hard part) but if it is, and if you're not comfortable rewiring your attic, there are track systems that have cords, and cord guards can be basically stapled/glued to the wall. Look for a Firefly or Valo instant track lighting system. (
http://www.westinghouselighting.com/litesnow/lighting-firefly.html or
http://www.smarthome.com/4624f.html) You might want to budget for a skylight on a south facing roof, but that's pricey. Fireflies are much cheaper.
Use built-ins or semi-built-ins as much as possible (bookcases, desks, etc.) One of the greatest desks I ever saw was made out of some 2x4, some sheets of chipboard and a can of polyurethane (or fiberglass epoxy resin). The owner converted a walk-in closet to an office so he ran 2x4 around the edge of the room at desk height, screwed the chipboard into the 2x4s, then coated the whole thing in polyurethane a few dozen times. Pics and descrip are here:
http://www.simplebits.com/notebook/2004/03/25/particle_board_desk.html Here's a pic:
You could also use a door, an old window, or a piece of rescued countertop and some filing cabinets for workspace. Remember to put them under the slant, and leave the middle, headroom area as clear as possible.
If you must buy furniture, get the smallest furniture you will be comfortable with - this is a place for a rolling laptop stand or an escritoire, not the huge honkin' Mile O' Mahogany. It's easier to adjust the height on a desk than the square footage, and it's easier to stash papers and desk stuff in drawers and files than to get a big desk up the stairs or through the window.
Go to the library and look at design books that are geared toward small spaces. Don't forget art, either - there are hangers to mount pictures on slanted walls, and a couple of pieces can give a lot of visual interest.