(For web applications, you might want to consider using the AGPL instead of the GPL.)
Assuming that the whole software was created by your team, you should follow these steps to license the software under the GPL:
- Add a copyright notice to each source file.
- Add a copying permission statement to each source file.
- Add the full license as plain text file to your project.
For details, read How to use GNU licenses for your own software.
On top of that, you might want to state that the software is licensed under the GPL in other places, too:
Mention it in the user interface of the software. For example, under "Help → About" or in similar places. It’s likely that not all users read the source files, and sometimes users end up using your software without having installed it themselves – these people might not be aware that it’s free software.
Mention it on your website. Ideally on your homepage, as feature, next to the download link etc. It can be a big advantage, and many people won’t even consider using software unless it’s free/libre.
(Many web applications that create websites also include the software name/link and the license information in the footer of the created websites, acting as some kind of advertisement. But it might be nicer to make this opt-in instead of opt-out.)