2 things:
First, every freshman should try to have a good grasp on the role of the entry system, which is such a big part of freshman year at Williams. Each entry is unique, is partly shaped by the JAs, but is mostly shaped by the freshmen in the entry. Some freshmen need the entry more than others; it becomes their main social scene and their main way to make friends in a way that's comfortable for them. Others are much more comfortable going out on their own and making friends outside the entry, though this does not necessarily mean these freshmen won't be a part of the entry at all. Still, some freshmen might not want/need to be a part of the entry at all and may distance themselves from it, or become close with a different entry that they might feel closer to.
As someone who is going to be a JA to the Class of 2016, I believe it is important for freshmen to understand that no matter where you fall on this spectrum, the entry is supposed to be a safe place where you can unwind and feel welcome, even if it is not a place where you would like to spend most of your time. Freshmen often get their hopes up that everyone in their entry will be best friends with each other, and then are devastated if that doesn't happen. I don't want to completely throw a wrench in this: many entries do become extremely close and stay close throughout college--two of my closest friends now were in my entry freshmen year. But it is important for freshmen to understand that while this can, and certainly does happen, it is by no means inevitable. Instead, freshmen should take the entry as is, putting themselves out there and attempting to grow close to and be a part of the entry, but also realizing that not fitting in with their entry does not equate to not fitting in at Williams as a whole; rather, it simply means the freshmen need to find their own niche, either by getting involved with a club, making friends from class, or growing close with other entries. Finding your niche at college is arguably the most important thing in making a good college experience, and the entry should be seen as something that facilitates this rather than hindering it.
Second (and more briefly), freshmen coming in to Williams should understand how important it is to ask for help, especially academically. In some of my other responses I mention the various academic resources available to students. It is really important to use these. Because of the highly intelligent and competitive nature of Williams students, it is important for students to realize that other students struggle as well and that is it not only okay, but encouraged that students ask for help. It is also important that students, when necessary, are willing to address the stress of their academics in a healthy way, by talking to friends, JAs, Peer Health, or even Psych Services.