Welcome to the forum Lindsey.
You have a tank that contains mostly common livebearers with a few added fish, the silver sharks and the betta. In that situation, you will do best with a good quality name brand fish food such as any of the major manufacturers provide. That will take care of the staple food needs in your tank. I always supplement my livebearer tanks with extra veggies in the form of a green flake food.
As far as compatibility, I am a bit concerned with your choice of silver sharks in your setting. The rest of your fish will function fine as a community but the sharks are a bit aggressive for that mix and may cause trouble over time.
The cleaning question is easy and again not so easy. If you have a fully cycled tank, a simple water change of 20% to 30% weekly using a gravel vac to clean your substrate will keep everything healthy in the tank. If you do not yet have a fully cycled tank, you will need to do larger and more frequent water changes.
We have some good articles here on TFF that will guide you through a fish-in cycle if that is where you have found yourself. You can reach the main articles on fishless and fish-in cycles using the links in my signature area.
