OpenMRS seems to be a newbie-friendly OSS project.
E.g., they are participating in the Google Summer of Code where young developers are supported to develop real-world features with the help of mentors. See projects of this year:
They also have a nice developer guide and the remarks on one pull request I had a look at a few month ago where very friendly towards someone who seemed to be on beginner skill-level.
Here they are listing opportunities for contributions:
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