The general consensus is RODI water is the best. It gives you basically pure water. You add a good salt to it and it now contains all the minerals that you need for a reef tank. As far as water filters go, you can buy a water filter to purify your tap for better drinking water. This will help to filter out chlorine, heavy metals, bacteria, etc. but it won’t demineralize your water unless it promises to. Just need to read the instructions.
Sounds like you’re still new to reefing. I recommend reading a bit more. Watching some videos (BRS TV on YouTube is great). And continue to keep asking questions. Generally people on this forum are helpful, won’t criticize you for making mistakes and can give you lots of info to help you along.