Just a couple of comments... I'm a rising 2L, and was lucky to land a firm job this summer in Boston. I only know of a handful of classmates that are working at large law firms this summer, and none of them had any prior law-related work experience. That said, all (except one) of them did have at least two years of prior "corporate-related" work experience (e.g., consulting, financial services). So while I don't think prior law-related work experience is necessary to get a summer firm job, those with some work experience were certainly at a comparative advantage.
Regarding resources at the school, you'll find that as soon as school starts, the CDO will tell you, "Don't worry about finding a summer job, focus on your class work." In the main, they're right. Your first-year grades are much more important in the long run -- employers seeking to fill their 2L summer positions focus a great deal on them. But it's hard NOT to worry about finding a summer job because, after all, you'll have to find some way to pay the rent, buy groceries, etc. Fair warning, you should expect little help from the CDO in finding a job for the summer after first-year (aside from the occasional resume review or panel discussion). You're largely on your own. That means, start networking early and often, set up informational interviews, and leverage personal contacts, etc.