The simplest way to load ERC is to add something like this to your ~/.emacs file:
(require 'erc)
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.
(require 'erc-auto)
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.