but given the fact that I bought the used wii, how can I start the mod attractive procedure? can I format the wii and start making the change welcome? and others where to find a guide to download the games and put them in the SD?
I don't recommend formatting the Wii via homebrew; as the NAND (storage) would very likely be corrupted. If you believe you must factory reset, make sure you back up anything you want to save to the SD card.
However, that may not be necessary. Do a search through your favorite web search site (Google, Startpage, DuckDuckGo, or what have you) for Complete Softmod Guide; it should be a Google Sites website. Also, you can go to WiiBrew by simply typing "wiibrew.org" in your web browser's URL banner. You should be able to find all the modding tools and instructions you need on both sites.
By the way, what games have you got, and how did you install them?