Every attic is different, but a few established norms and practices will guide your renovation.
Follow the "rule of 7s": Enforcement varies, but codes typically say that at least half of a finished attic must be at least 7 feet high, and that this area must be a minimum of 7 feet wide and 70 square feet. A contractor or a local building official can help you assess how the rule will apply to your attic and how modifications like dormers can resolve height shortcomings.
Have a pro check the structure: A finished attic weighs a lot more than boxes of off-season duds. Hire an engineer to inspect your house's foundation and framing to ensure they can carry the extra load. At a minimum, you may need to strengthen the attic's floor joists, which are often too shallow or spaced too far apart for the job.
Assess your access: If you're building a stair-case from scratch, consider a switchback layout. It needs more room than a straight run (roughly 45 to 50 square feet per floor versus 33), but its footprint is more squarish than linear, so it will often fit in spaces where a straight run can't go. Just make sure the landing is large enough to maneuver furniture upstairs.
Pictured: A focal point at the top of an attic stairway, like this peaked bookshelf, draws people up.