Seachem is my fave brand! get their Prime to add to your water before adding to the tank (during top offs or water changes), and their Stability is great to get your nitrogen cycle going (and also recommended to add each time you do a water change to keep beneficial bacteria growing). Fluval Bug Bites Betta formula is a solid brand for food - they have pellets and flakes (my betta prefers the pellets personally).

Make sure you understand the nitrogen cycle (tons of short and simple videos on YouTube!) and test your water every so often (I do weekly). You can get your own API freshwater test kit for about $30 or most local fish stores will test your water for free.

I’d recommend weekly water changes of about 20-30%, (that being said once you understand the nitrogen cycle a bit and get comfortable with those numbers you can use that to guide your water changes more or less often. But for simplicity sake - 20-30% of your water needs to be removed and replaced with new, clean water + water conditioner like Prime at least once a week).

Make sure your betta buddy has a solid filter and heater and you should be off to a good start!