Need advice for a good betta fish tank

Hello there! I know there are a lot of lists about the best things you can get for your betta fish, but here's the only problem I encounter:

- I plan on getting one betta fish and another species of fish in the same tank

- I only want fish/animals in the tank that can comfortably coexist with the betta

I'm comfortable with spending some extra money on this. I just need to know the recommended size of a tank, decor, and other helpful information, ESPECIALLY what other species would comfortably be in the same tank as a betta and simulate a good environment for them.

I am very new to fish, but I LOVE fish, and I love animals (I've only ever worked at a cat/dog shelter). Seeing fish being so abused in stores makes me very upset so I really want to give one a good home. All advice welcome, please!!