erc-autojoin-channels-alist

erc-autojoin-channels-alist is a variable defined in `erc-join.el'.
Its value is

nil

Documentation:
Alist of channels to autojoin on IRC networks.
Every element in the alist has the form (SERVER . CHANNELS).
SERVER is a regexp matching the server, and channels is the
list of channels to join.

If the channel(s) require channel keys for joining, the passwords
are found via auth-source. For instance, if you use ~/.authinfo
as your auth-source backend, then put something like the
following in that file:

machine irc.example.net login "#fsf" password sEcReT

Customize this variable to set the value for your first connect.
Once you are connected and join and part channels, this alist
keeps track of what channels you are on, and will join them
again when you get disconnected. When you restart Emacs, however,
those changes are lost, and the customization you saved the last
time is used again.

You can customize this variable.