The simplest way to load ERC is to add something like this to your ~/.emacs file:
This will define the erc-select command used connect to an IRC server.
To prevent ERC from loading at startup, you may use auto-load in your ~/.emacs file. An automatically generated library of autoloads comes with ERC, and can be loaded with f.ex.
If that’ll lead to too much clutter or you have other reasons to define the autoloads yourself, you may of course do so. Evaluating
(autoload 'erc-select "erc" "IRC client." t)
will declare an autoload for the most useful ERC command,
erc-select, which is the fundamental way to start an ERC session by connecting to a IRC server.