Find an existing project that's already in your area of interest - e.g. a sporttracker if you're into running - and start contributing to that. No point in starting someplace that's completely alien to you.
Preferably, pick an existing project that you already use in your area of interest.
I recommend you do it. /s
In seriousness, I think you need to be more specific about what you want to know/understand, your question is pretty open ended.