Much like any other job search (inside and outside of medicine), you need to have a focus before you search. In medical research, that is broadly: basic research or clinical research. This is a given because either of the two teach you very different sets of skills, which likely helps you much more so (and is a better use of your time) for preparing for residency programs.
The second important decision you need to make is what specialty do you want to get into. You may not know, but I'd suggest getting that squared away before you go into research. Research for the sake of research (and your CV) is usually a bad, unsuccessful approach. Research isn't something you do to mark it off on a checklist. Figure out - even broadly what kind of research you want to do. Oncology, perhaps (as that could apply to a wide range of subspecialties down the line, primary care, oncology, radiology, surgery, etc).
Once you figure that out, seek out the established labs that have good mentors. Good research is often proportional to the ability of that PI to mentor and provide guidance to good research mechanics, as well as having good research projects/grants already on-board. Some people flock directly to the famous labs (see Nobel Laureate Eric Kandel and neuroscience at Columbia). You can argue the right and wrong of that, but if you just want to move up the medical ladder, that approach is an option as well.
All in all, I'd strongly suggest you answer the question of where you want to go in medicine before you decide on a research opportunity. The investment in thinking that through now will pay off later.
