Lynne, the first thing you'll want to look at is where you want to lose about a 4' X 10' area of ground level floorspace to accomodate a traditional stairway. Likely part of this area will already be taken up by the spiral staircase, but the new stairway could also be in a completely different area & you would recover the floorspace of the existing stair.
Look at your main floor and determine which direction you'd like, then look at this same area from the basement standpoint, but add a few more feet in length (since the actual stair will cover more space on the basement floor than the main).
Preferably, the stairway would run parallel to the floor joists, but it can be done either way. Look for what might have to be relocated to accomodate the new stair, and what cannot be relocated without a great deal of trouble & extra cost. If this stairway will begin on the main floor near the center of the home, descending towards an exterior wall, be sure there will be adequate room for a landing left at the bottom of the stairway.
An average stairway to a basement will cover about 12' in length, and 3' in width, plus the width of one wall, if the stair is placed beside an existing wall. The actual total distance used will depend on the celing height in your basement, and what height risers you choose to have.
You can also have an "L" shaped stairway, but you'll use more floorspace doing it, so keep this in mind.