Be sure to shake the bottle well before measuring out the dose. Give after feeding, and repeat in 2-3 weeks. I've always found Strongid to be very effective. Droncit tablets kill tapeworms, hookworms, roundworms, and whipworms. You have to get these from the vet.
Dogs get tapeworms by ingesting fleas.
The best way to prevent tapeworms is to prevent fleas - an application of Frontline Topspot or will kill fleas very quickly after contact with the dog for 90 days. But Frontline kills ticks for 30 days as well, and is now available over the counter so you can find some pretty good prices at pet supply stores and online sites. Hope that helps.