Good idea and well done for thinking further in the field.
A couple of suggestions:
Open source technologies often involve groups of programmers working together to create, maintain
and modify software that other people (companies) have worked on previously, in exchange to use
any service they provide - typically for free.
A good suggestion would be to Google "open-source Java Projects" and see what turns up.
As for beginner friendly - I believe there is a website called "planetsourcecode.com" that has
(or did have - have not used it for a while) an option where you can not only learn about
open-source from looking at other people's code that they post - but you can message them
and ask to be a part of it.
As for mentoring - it would probably be a mixture of the first two. If you get on a site or
meet anyone within the open-source community, make a good friend or more first, share your
ideas and experiences with them and work together. In fact, you could be a mentor to another person
with less programming experience than yourself.
One final idea :- If all else fails and you really cannot find a foothold - you could always begin your
own project in say Java, make documentation about it, and post it on a forum which asks people
if they would like to help you on the project. This is a good way to get noticed in what can be pretty
tight niche' of a field to get into. Most professional open-source projects and handled by large teams
of programmers with vast amounts of experience - but that will only spur you on to become on the
Good luck in your quest.
Wishes Ada xx