Ask HN: How to overcome hurdles of first engineering internship?

Hello wise engineers. I just started my first "real" job as a software engineering intern. I'm a CS student, and in school our projects are typically quite small (up to ~2,500 LOC) but I'm now working with one repository in the order of 100,000 LOC. My supervisor is sympathetic to me being overwhelmed, but I'd like to dive in and get hacking as quickly as possible.

What would you suggest a new intern do to get acquainted with a large code base, considering s/he hasn't worked with one before? And/or more generally, what advice would you give to first time interns looking to make a good impression? What valuable lessons would you pass on?