Sure it's open but theoretically nobody is allowed to use your code. To attach a license is really simple. I recommend the GPL so this is what I'll guide you through:

1. In your project folder just create a file called LICENSE.md. Most platforms like GitHub or GitLab will pick up on that and even show that your software has a license when people visit your repo.
2. Copy the content from https://www.gnu.org/licenses/gpl-3.0.md over to your LICENSE.mdfile.
3. Now people can use your code in their projects.

If you look at my projects on github you'll see that all of them have this file and all are licensed under the GPL.