Firm's own website
They are pretty much all the same. Absolutely know which cities they are in, approx how many lawyers, where the interviewer(s) went to UG and LS, any recent news about the firm, any recent HUGE cases the firm won, and read or skim anything the interviewer has recently written.
Although you probably shouldn't talk about it in the first interview, you should probably also be aware of the the billable hour requirement and starting salary. Also, if you think you may want to split your summer, you may want to look into that at this stage.
For BIGLAW you can knock alot of this out on NALP.