Setting up a tank and choosing the perfect betta is the most exciting bit. :) I wish I was in your shoes...again!
You may have an idea of what sort of betta you want, but be warned - falling in love with bettas is an unpredictable process. When my first girl Aphrodite died, I went into the petshop set on a gorgeous white delta I'd wanted for ages , and came out with a grumpy, sassy, bossy little blue girl who absolutely captured my heart. Go with the one you fall in love with, and even if they are the ugliest, scrappiest little thing you've ever seen, you won't regret it.
The tank that BettaMommy suggested does look good, as long as you follow her advice and only keep one betta in there.
There are a few different substrate options. Gravel is very easy to keep clean and rinse during water changes, so I'd suggest that over sand (sand can look amazing, but personally I think it's a pain to clean, and you need to do 100% water changes on a tank that small...sand would be a nightmare).
When it comes to decor, make sure you look out for a few things:
- don't get anything with a rough surface
- don't get anything with an exposed wire
- no sharp edges
Silk plants or, even better, live ones, are great for betta tanks. I've always preferred live, and plants like java fern, anubias or java moss are just as easy to care for as silk, IMHO. Basically, stick them in the tank and they grow. They also help consume nitrates. :) Anubias is great as bettas love resting on the broad leaves. You don't plant it - you just tie it to something, and it roots on. A small piece of driftwood, or, better yet, a small terracotta pot, would be perfect, and a pot would also double as a cave. Many bettas love having caves to hide in and explore. :)
Good luck with your new betta, and pictures pictures pictures!