You'll need:
A co2 tank: choose paintball or regular. Paintball tanks are smaller but cheaper initially and easier to find a shop (Dicks sporting goods or other sport stores) to fill. Normal tanks are harder to refill but welding supply stores will. Make sure it fits on your regulator or you have an adapter. Any brand works really. Mine is an empire paintball tank.
Regulator: goes right on the tank (needs to fit, some work only on paintball and vice versa) and decreases the output pressure. Also most importantly lets you adjust the flow. Milwaukee and aquatek brands are good, GLA is super high quality and worth it if you have the money.
Tubing: co2 resistant tubing, make sure its aquarium safe and getting stuff thats the same as your tanks background is super helpful, trust me. Any aquarium and co2 safe brand is good.
Bubble counter/check valve: sometimes they're separate but finding a 2 in 1 is easy and helpful. It ensures no water goes into the tank (check valve) and lets you count the bubbles (bubble counter). Fluval has a good one i think.
Diffuser: you can just run the co2 line right into the intake of your filter, or get a diffuser. The first would be cheaper. Air stones aren't good enough IMO, so a proper ceramic diffuser or atomizer is helpful. Some are super expensive and you pretty much get your money's worth. There are some generic ceramic diffuses on amazon, cheap ones are fine and work well enough.
Co2 drop checker with 4dkh fluid and pH reagent: this tells you how much co2 you're pumping in. Green is good blue is too low yellow is too high (can harm fish, invertebrates are particularly sensitive). Any brand on Amazon works pretty much. Glass is good to have, and looks good. Its pretty self explanatory.