There are two good ways to do this:
You can simply use remove_column, like so:
remove_column :users, :first_name
This is fine if you only need to make a single change to your schema.
You can also do this using a change_table block, like so:
change_table :users do |t|
t.remove :first_name
end
I prefer this as I find it more legible, and you can make several changes at once.
Here's the full list of supported change_table methods:
http://apidock.com/rails/ActiveRecord/ConnectionAdapters/SchemaStatements/change_table