I would search this forum at the bottom of this page for 'dahlia storage'. There are a lot of great threads on here.

If you still have questions come back and post again.
Keriann~

I would keep it in the ground until it frosts, cut off the foliage, then a week later dig it up, wash it, dry it, and store it in peat moss in a cool place. Some people do fungicide. I didn't last year do fungicide last year, but may this year.
Good luck!