2 options:
Directly into the HTML code:
<p style="color: red; font-size: 50px;"> text here </p>
Via a CCS file:
HTML
<p class="large-red-text">text here</p>
CSS
.large-red-text { color: red; font-size: 50px; }
Just be sure to save your CSS in a file with the .css extension and include it in your HTML in the <head> tag like this:
<link rel="stylesheet" type="text/css" href="filename.css" />Â