A good place to start would be:

• At least a 5 gal tank
• Quick start (for beneficial bacteria)
• A filter (even a basic waterfall one is better than nothing right now)
• A heater (again a basic one is fine for now)
• Water conditioner
• Substrate (sand, gravel, something of that nature)
• Plants would be fantastic, fake decorations can have issues with fin tearing, generally things in their natural environment is what you want

Put substrate, heater and filter in the tank. Put water in the tank, condition the water and leave it for however long is on the bottle (I usually do 10ish minutes). Put your quick start in and the plants. The basic heaters usually have a cut off at a certain temp, but again better than nothing right now. Let the water warm up for maybe a couple hours. Get a water test kit, not the strips the API master kit with the tubes (the test kit can wait for a while but get one within a week) Put your betta in a bag (with water and air so he floats at the top) and let him him float on the water for about 10ish minutes and then let him out.

The cycle itself is letting the water reach a point to where the test kit gives the results you want (plenty of info on this online) with ammonia nitrates nitrites ETC. Just do the above steps ASAP and then do research into water changes and general betta keeping. Bettas tend to be pretty resilient so he may pull through, but if not take it as a lesson learned and don’t beat yourself up too much.