Dedicating yourself to a specific high quality open source project (popularity doesn't not necessarily imply quality, but in the absence of other indicators start here) can serve in many ways as a mentor would. Some advice on getting started on an open source project:
Open source projects (whether corporately funded or hobby led) are run by people! So it's in your best interest to give before you take. Become active on the communication channels, answer user questions, fix issues where you can, etc... Once the lead developers recognize your name, they will be more willing to review your code (and get your changes merged when review bandwidth is low). Well run projects often have very knowledgeable people that can point out mistakes and teach you in general.
Of course, there's still value in being a "drive-by contributor" where you fix a small bug and never come back. It's just unlikely that you'll form any sort of relationship from such an interaction.
Open source projects (whether corporately funded or hobby led) are run by people! So it's in your best interest to give before you take. Become active on the communication channels, answer user questions, fix issues where you can, etc... Once the lead developers recognize your name, they will be more willing to review your code (and get your changes merged when review bandwidth is low). Well run projects often have very knowledgeable people that can point out mistakes and teach you in general.
Of course, there's still value in being a "drive-by contributor" where you fix a small bug and never come back. It's just unlikely that you'll form any sort of relationship from such an interaction.