title: How to login a user using firebase authentication
body: <p>I'm currently using <code>firebase-admin</code> to create a  web Dashboard with <code>node.js</code> as a backend which will have multiple users and those users have user specific data in my database, note that I need them to be able to delete or add stuff to the database as well, I've managed to add new users to <code>Autentication</code> using <code>firebase.auth().createUser()</code> programmatically, but how would one go about logging in a user, and then from there controlling which <code>uid</code> is logged in and displaying his data (giving him access to the correct data, obviously don't want him messing with someone elses data).</p>
  
  <p>I know this might seem like a really newbie question, and it probably is,  but firebase docs always get me confused for some reason. Any tips? I'd greatly appreciate.</p>
  
  <p>Any questions don't hesitate.</p>