While this is not the first time for Riot Games, this incident is one of the more serious attacks.
Information at risk are first and last names, account username and password, email address, and credit card information. The identity of the hacker(s) is unknown at this point.
Everyone should receive a notification email regarding the incident. Affected account holders will receive a second email notifying their information has been accessed.
Switching into a more complex password through the game or website and watching your inbox is the only thing you can do at this time. Riot Games will be using email and SMS verification in the near future to prevent these attacks.