Hello!
I’m new to Defold, but I like so much his philosophy, style and community.
I’m 26 and I’m a software developer and I would like to contribute in some manner to the project…
Unluckily I never contribute to a project on GitHub.
How I can start? There is something of easy that I can do?
Have you any general suggestion? Common beginner mistakes that I can avoid?
Thank you! 
Welcome man, you can build some extensions for Defold or do some traslation work.
If you want to contribute to the engine / editor here is some info.
This is a list of issues that have been marked as good first issues for people who want to begin contributing.
https://github.com/defold/defold/issues?q=is%3Aissue+is%3Aopen++label%3A"good+first+issue"+
If you want to contribute to the editor make sure you’re on the editor-dev branch
Some of your questions are answered here, but you should read most of the readmes next to it too.
You’ll need to get the editor / engine building on your local computer and that will be the first hurdle you’ll have to overcome to meaningfully begin contributing. Some of the “good first issue” issues are probably relatively easy to do, but it depends on you too and how much you already know / are willing to invest into learning more of.
Thank you all for your suggestions!