Prepend the GPLv3 header in front of each source file
As suggested by @jonas:
Ideally you should read and understand the license you use (I guess that you did not - it is quite normal to not read the loooong *GPL licenses).
You can cheat, however, and read only the very last part, labelled "How to Apply These Terms to Your New Programs", which include a boilerplate text that you can copy-paste and adapt with your own name, authoring year, and project name+summary.
I recommend to add such text near the top of any source code file - i.e. right below what must be at top, like the hashbang of shellscripts.