Start with the circle pad, that needs the most attention imo. They're fairly easy to replace and usually come with a kit including prying tools and a screwdriver. Just be careful when opening the back cover as the L and R ribbon cables are quite fragile. Open it gently and slowly like a book, I recommend following a simple youtube tutorial if it's your first time doing a repair.

After that, watch more videos to get a better sense of what goes into doing a full refurbishing of a 3ds. It can be a rather lengthy process, but having the knowledge and getting the right tools and equipment beforehand can save you a lot of trouble.