It's often good to have more than one mentor. You should have at least one mentor in the field you are going into -- this person will be able to point you toward opportunities, advise you when you are troubles relatively specific to your field, introduce you to other people in your field, etc. I'd also try to get at least one mentor who you trust but is completely outside your field, so you can get an outside perspective as you become increasingly buried in your own specialty.
As far as formality, it doesn't need to be formal. I've found it works best with attendings who can tolerate a little informality in the relationship, in fact... best if you can just stop by the office rather than having to make an appointment eight weeks in advance (although you can make that work, too).
Just keep an eye out for people who have their heads screwed on as you go through your medical education, and stay in touch with them as you move along. You may not ever use the word mentor with them, in fact.
Best,
Anka