Starting Out
First off, let's talk about what every application will need to do to actually work.
Initializing and Connecting
Once you have required the Eris module, you need to instantiate a client, and for that you need a token, found on your application's page. It's strongly recommended to store your token in a separate file, and load the token from that, instead of writing it into your code directly. You can also supply the constructor with an options object, specifying values for certain options. You will most likely not need to use them, but they're there if you do.
var eris = require('eris');
var auth = require('./auth.json'); // One way to save your token
var client = new Eris(auth.token);
Or, with options set.
var client = new Eris(auth.token, {getAllUsers: true});