Hello there Betta are amazing little fish. First off it would be a great to go to this link and read the information there to find out about Betta fish and their proper care.
By reading from this link I think you will find what you need for your Betta. If your going to get a tank with a filter its always a good idea to set the tank up before you get your Betta and let the tank run for 24 to 48 hours before putting your Betta in the tank, this is to make sure everything is running right and there are no leaks.
Getting your supplies together is up to you mostly but buying ahead of time would be great so you will have what you need such as food and water conditioners when you bring your Betta home. Prime if a very good water conditioner and something you will need to add to your water and also you need to add it for every water change.