These two stars are a case of the myths fitting the reality, since Pollux is the brighter (17th brightest) while Castor languishes at 23rd. Since many versions of the Classical story of the Heavenly Twins made Castor the mortal one, it seems fitting that his star is slightly dimmer.
Rigel is basically Orion’s ankle. You can find it if you look for the three stars of his belt, then down the belt to the bright white star below it. The name comes from Rijl Jauzah al Yusra, the Left Leg of the Jauzah (Jauzah was the Arabic title for Orion).
Fun fact: Rigel was mentioned in several Star Trek episodes, in the form of its planet, Rigel VII. Captain Pike had landed there and been attacked, and this history, as well as more current events, were mentioned in episodes of New Generation and Deep Space Nine.