Before you build a spiral or circular stair, you should check with local building department for code complaiance. Most stairs must meet specific width and tread requirements. Spiral stairs have nesrly zero tread width near the center support column and are usually required to have 7-1/2" tread width at 12" from the column. These stairways are fairly economical to buy as pre-fabricated units.
If after checking code you want to tackle this, I am attaching a web-site that offers plans, but I would personally look to buy pre-fab. Spiral stairs are primarily supported off a central column, proper construction and structural integrity is vital. Hope this helps, luck.