We used to show icons depends on images. But it’s not a good idea because we can’t easily change their color and size. Nowadays icon fonts apply to the web design. So we can easily change the icon’s color, size, even shadow, etc. via CSS, just as font.Here I recommend some popular icon fonts for you.
It is maybe the most popular icon fonts now. Because it is free, lots of icons for selection, and keep updating.
Dashicons is the official icon font of the WordPress admin as of 3.8.
You do not need to place the dashicons files to your theme or plugin. Because it has contained the WordPress core diretory. So you just add the code below, then it works.
<?php wp_enqueue_style( 'dashicons' ); ?>