I learned how to be a machinist from the owner of the local tool shop who was an ex-tool and die maker. I learned systems programming from various helpful people in IRC and people who have advised my internships. I learned physics from other grad students and my professor.

Just make friends with people who have the bandwidth. I also strongly disagree that mentors are overrated. They're necessary in some disciplines, and helpful in most. Learning without a mentor is like running open-loop. As a new student, you often won't even know the right questions to ask. How will you know what to google? How will you know if you're developing a bad habit?