If you are getting a 5G or bigger tank and have any plans to cycle the tank, a water test kit is a good idea. The API master kit is what many of us use; it is more expensive, but it will last you a long time.
For food, your fish will be healthier with pellets than flakes.
You don't have to have substrate but it's easier to hold plants down with gravel.
I have a small net as well which is handy for fishing things out of the tank on occasion, like floating leaves or uneaten food.
Happy shopping. :)