Say hello to your new best friend: http://www.tropicalfishkeeping.com/b...are-faq-20058/
If your bettas have not done to well int he past, I am guessing
a) Their tanks were not heated.
b) Their water wasn't changed often enough.
Hopefully the care sheet wil help. The only thing I can add is that as far as Aquarium starter kits go...
- Anything under 2g is a piece of junk that doesn't suit the needs of a betta.
- The bigger the tank, the less you have to clean it :)
- Eclipse Systems are awesome, but their filter is just too strong for a betta. The current it produces catches on a betta's fins like wind on a kite and blows them around.
- The Minibow (available in 2.5g and 5g) is an awesome tank kit for bettas.
And some other tips
- Never buy or do anything based solely on the advice of a fish-store employee! They more often than not have no clue what they are talking about!
- Live plants are a great addition to the tank, just be sure to purchase very low-light plants... the medium-high light species will pretty much flop unless you have fancy lighting.
- A betta' stomach is the size of his eye so feed accordingly.
- Even using different brands of food will vary your betta's diet quite well.
- Stay away from freeze-dried. They don't make a good staple at all and should only be used as a treat.
It would also help if you read up on cycling (the process of establishing a nitrogen cycle in your tank to detoxify waste.
Hope that gets you started and welcome to the forum!