Buying a Betta Fish shortly, and have a few questions?

Well I am going to my local pet store to buy a Betta Fish. I want to get a healthy one. What are some signs of a clean, healthy Betta? Also, I want to know what kind of fish bowl to put him in? Round or Hexagon shaped? Rocks or no rocks? What is some information I need to know? I haven't had a pet fish in YEARS! His name will be Poseidon! :D