ok. you are new to dahlias; I sugest you do some searches here. you will find great info.

In short, generally either there are no seeds or you wouldn't want them anyway. The plant stores its energy in a tuber underground. Cut off the stalk to 2 inches and carefully dig it up and get all its parts. This is to be replanted next spring. Store the tuber, cleaned off and dried, in an open plastic bag (keep it open slightly) packed in peatmoss, spanish moss or the like. The point is to retain the moisture content of the bulb, without letting it rot. Be sure to store in a cool dark place. Temps around 40-50 F.