Everyone always recommends a mentor, but I think most try to over-formalize the relationship. Also, we learn best from mistakes, anyway. If I try to learn everything up front from a mentor, I would never get started since there is no step by step guide for each specific deal. There are multiple ways to get a deal accomplished and everyone will likely have a different opinion.
I didn't find professionals to help me at first. I asked around at my 9-5 and found several folks who owned rentals. I picked their brains over lunch and started networking (got recs for contractors, realtors, etc.) and then picked a house and bought it. Those were my mentors because they started out as I did. Of course, I was doing SFRs, so no need to find a professional.
I made a bunch of mistakes, but it was a great way to learn. In fact, we learn more by making mistakes than we do by achieving success. Think about it, if you succeed, you're still always going to try to figure out how to make it "better". Its human nature. Eventually, if you keep tweaking what you did, you'll end up screwing up and then learn "not to do that again". I figure its better to screw up now with my $50k house than to screw up with a $50M apartment complex.
Once I had a couple of deals under my belt, then I sought out professional mentors through our local REIA and word of mouth. We still don't have a formal relationship; I just expanded my list of folks I can ask random questions to.