New Validator candidates should ➡ follow the process to become a candidate outlined here. You will need the minimum amount of STAKE as well as a working node configured with your mining address. It is best to start at the beginning of a new epoch to give yourself enough time to get everything setup properly.
You can choose to run either a Nethermind or OpenEthereum client. The Nethermind implementation is the recommended choice, Open Ethereum is currently being deprecated and future versions may or may not support POSDAO.