This course on Wi-Fi hacking elaborates the various techniques by which you can hack the WEP, WPA/WPA2 and WPS Wi-Fi routers. This course will walk you through the processes and steps right from scratch. At the end of the course, you will become a pro-Wi-Fi penetrator.
The kinds of hacking that are illegal are (depending on your specific jurisdiction) are "theft of services" or "unauthorized access to a computer system" or "fraud." It is perfectly fine, however, for someone to hack their own computer or website in order to reset a lost password for an administrator account.