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Hello Pedro, and welcome from me too 
With PHP / MySQL experience you can definitely join here! Drupal itself can be a bit daunting compared to less complex systems like WordPress, but not all tasks require extensive Drupal experience. I have filtered out those tasks for you that require no or only limited, specific Drupal knowledge. Means, things you can do after learning half an hour about the Drupal concepts involved, if any:
- 5. Exchange the markup creation JavaScript code with Annotator.
- 6. Implement exporting of selected codings to a CATMA file.
- 15. Improve the CKEditor styles to improve content creation for study participants.
- 16. Fix cut&paste changes to formatting inside CKEditor.
- 17. Improve the look of content created by study participants by limiting their use of HTML.
- 18. Prevent empty lines at the end of content.
- 20. Implement live-syncing selected codings with a certain account at a CAQDAS software.
(Only numbers and titles, look up details in the original post above). I would propose you start by installing a “vanilla” Drupal system on your computer, then take on one of the tasks not related to eComma (15-18), and after that proceed by installing eComma, learning some more Drupal and doing other tasks. If you get stuck, we offer to help out via pair programming. Let us know how you want to proceed