How to choose a mentor on drupal.org
There is a posting from Dries Buytaert titled Drupal mentors needed, where he basically asks senior people to become mentors ("Find, motivate, guide and empower people to take on a role within the Drupal community").  So if Dries is the official voice of the Drupal community (and I suppose he is), you're supposed to hang around until some aspiring mentor finds you :-(.  Unfortunately, this very rarely gives the prospective student access to a mentor.

There has been some attempts from other community members to improve on this situation.  For instance, there is the Drupal mentor group. with first and last posts in October 2010 (and one spam post from 2012).  You could try to introduce yourself there, but the group seems to be dead.

There is also Drupal Kitchen.  There is a handful of people signed on to the site as tutors, but to me, this site looks like yet another failed effort.

If you want to learn how to contribute to the core, there is core contribution mentoring. It uses @drupalmentoring (twitter) to set up office hours and sprints for mentoring aspiring core contributors. This may be educational, but is very much oriented towards the core contribution workflow, and not suitable for learning about Drupal in general-

Here is what I suggest:  Start by getting a user account on Drupal.org and Groups.Drupal.org (unless you've already done so).  In Groups.Drupal.org, look for groups in your geographic area. Join those groups and try to find someone local to communicate with, and look for local meet-ups and sprints you can attend.

Also, use this site (Drupal Answers), the forums on Drupal.org, and IRC to ask specific questions about the problems you have developing.  You will not get a specific mentor, but you will usually get relevant help from the community.

However, in my experience, when developing a module, the best way to get the attention from senior members of the community ("mentors") is to join the Review bonus program.  This requires some effort on your part, since you're supposed to mentor others (by reviewing their modules) -  but you learn a lot from doing so!  Do not be afraid of being a novice reviewer.  You'll get feedback from the other reviewers about your reviews.   It is OK to make mistakes, and you'll learn from them.