I didn't have any prior to starting medical school; research mentorship varies among schools and even within the same school.
My advice - as someone who fumbled his way through research during medical school - is to start planning your summer project for between 1st and 2nd year in November of your 1st year, because summer fellowship deadlines are as early as mid-January and possibly earlier. It can be as easy as setting up a meeting with someone in a field in which you're interested and then flat out asking, "do you have any projects I could help with or start?" Hopefully, this is a project that is good enough for you to carry it through 2nd year and get a solid publication.
Clinical research is much easier and faster to publish than basic science research.
Having said that, Step 1 and grades are definitely the most important. However, a few of my interviewers have mentioned that it's really good to have a first author paper.
Best of luck.