Okay, so you’ve decided your ready to edit your WordPress files. If you’re not sure where to start, Carrie Dils has some helpful blog posts on how to do just that:
Editing Your WordPress Site 101
Teach Yourself to Troubleshoot
If you prefer not to edit your style.css file directly, you can add CSS in the Customizer at Appearance -> Customize -> Additional CSS. You can just cut and paste your code in and save it.