Don't cheat and look up the secret codes in the Gravity Falls episodes. Sure, you can look up where they are to find within the episode, but don't actually look up what they say.

     I mean, wheres the fun in that?

     Instead, here is all you need to know:

Episodes 1-6 use the Caesar cipher, episodes 7-13 use the Atbash cipher, episodes 14-19 use the A1Z26 cipher, episode 20 and 31 uses a combined cipher (a combination of the A1Z26, Atbash, and Caeser ciphers), episodes 21-30 use the keyed Vigenère cipher.  

All credits for that go to the Gravity Falls Wikia.

Thank the interwebs for such a beautiful place. It's where I go when I forget any of what that Bill Cipher has ever said so that I have all of his lines forever memorized.

Because that is my main priority in life.