We need to have a model for onboarding new contributors that helps guide them to issues / activities that are small enough to bite off. Each new, prospective, contributor should have a mentor that is primary, but all communications should be via the public forum / issues etc …
Other thoughts? Training assets, guidelines, style, etc?
Guideline #1. Call it Boarding, or Embarking, not “onboarding”. Please. After they have boarded, they are aboard, and they are embarked. Just like in an airplane or passenger liner.
Do we need disembarking as well?
how about just Barking 
Try “Deep Sixing” in those cases. It is a better clean break, and you don’t having them coming back aboard for forgotten luggage.
Ceasing the hijacking of the thread, and onto the subject: We need afford the contributors (note spelling in thread title) a flowchart of some sort that identifies their interests and capabilities before pointing them to project resources like coding guides and task lists.
Perhaps a google form / questionnaire?
If they aren’t “Googlers”, then perhaps it may be better to use the tools on this site?