She's cute! I love the red and white ones! Glad that a fellow fish-lover got her instead of someone else. I've heard horror stories of fair goldfish being stomped to death in parking lots! :(
You're right in thinking that goldfish shouldn't eat betta food as their main diet, but for the first year of their lives, they need a lot of protein akin to what is found in betta food. For right now, it will do just fine. Goldfish also enjoy green veggies like red lettuce, kale, and spinach. If you have any extra laying around she would enjoy it.
For basic care, there are a couple things you should know. Goldfish get big. Goldfish are messy. And goldfish are social creatures. The long and short of it is that your little goldie will need a 55+ gal tank to be happy and healthy long-term. Truly a pond would be best. These are just some of the reasons I don't support live animals as fair prizes. They essentially give you a long-term responsibility.
This is the goldfish thread we've started here. It's mostly for fun, but it does include some care tips beyond what I've told you here. Plus it's just a fun place to share goldfish pictures :3