Talk to your building department about local building codes. There may be requirements for headroom, access, floor space, egress and other items.
Check the structure of your attic. Make sure the roof framing allows for enough usable space, and the floor joists are strong enough to hold people and furniture. Have the attic checked out by a building inspector or structural engineer.
Install a stairway if you don't have one already. Pull-down stairs are probably not permitted by your building department. A spiral staircase may be acceptable.
Get quotes from electricians and heating contractors. Add electrical lines and bring in heat and air conditioning ducts.
Add insulation. Install drywall over framing and ceilings. Prime the walls. Paint or add paneling.
Build dormers, skylights or larger windows to let in more light. Check the path of the sun to make sure skylights and windows are installed in areas that let in sun at the best times.
Choose flooring that is easy to clean. If you choose carpet, create closet space with room to hold a vacuum cleaner so that you don't have to drag one upstairs.