Font awesome is a library of icons compiled to help you make integrate icons in your project very fast, easily and without a need to store or edit images.
Now once you register, you can only use 1 free kit to integrate, into your website which comes with 1500+ icons, so that does give you all what you need.
Now once you open the kit, you will have an integration URL, something like this.
We copy this URL and paste it into the HEAD tag of our login.php
Now lets get to font awesome icon and integrate an icon for our login button.
Click on the sign-in-alt icon
Now you see the tag, this is the code to call the icon. Paste it in between the button.
Lets save and refresh our browser
We have an icon. Cheers... :)