Installing Devise requires some effort on your part, which includes:

bundle add devise

Next, run the installer, which will add the initializer as well as the localization bits:

rails generate devise:install

You can find the initializer (with config settings) inside config/initializers/devise.rb. Our next task is to associate (or create) the model that Devise will work with:

rails generate devise User

If you don't have a User model, Devise will create one for you. Otherwise, it will add stuff to your existing model. Have a look and see the settings in there, and also make sure your migration aligns with whatever you choose (trackable, confirmable, etc).
Finally, we need to output the views so we can tweak them:

rails g devise:views

Make sure you run your migrations, and off you go!