Reach out to the organisation administrators and its community. Introduce yourself and tell them you'll like to contribute. Most open source projects are really welcoming and nice to beginners. They'll either assign someone to help you, or they will show you where to begin in the documentation. I'm not sure what level of experience you have with programming, so I can't really recommend any organisations. But just bare in mind it might take you a few weeks/months to fully get how the organisation code/community works (that depends on the org size/code base too tho)
Lastly, welcome to the open source world (if you were not in it yet :P)