Cosmos, one of my personal favorites, I grow these every year. Easy to grow from seed in trays, and like all seeds, pot on once large enough. In the right conditions Cosmos will grow quite tall and may need support once fully grown. 

To maintain flowering always dead head, if the dead flowers are not remnoved the flowering will stop. The flowering will comtinue until the first frosts which will ultimatley kill the plant off. As with all annuals, remove at the end of the season.