You really dont need much prior knowledge, though the more coursework you've completed, the better.
I started at a paid position in the lab cleaning dishes, taking out biohazard waste, autoclaving... once I started school as a freshman and by the end of the school year, I got kicked up to doing actual research. I didn't know a thing about western blotting or tissue culturing or PCR, so my supervisor taught me everything. You learn how to do the experiments and the you eventually start to understand what you're doing and why you're doing it.
I personally believe that the labs are looking for people who have good potentials to contribute to their lab, ie those who are hard workers, dedicated, willing to learn and diligent (you get the picture).