If you are a junior/senior then make sure you go to Career Services and get your CV (curriculum vitae) critiqued. If you want to participate in the the Roundtables you must have it done by 15 September.
Visiting Heidi Harlander (at SJU) is best as she is very familiar with jobs and how a CV should look, especially for STEM folks (Science Technology Engineering and Math).
An extremely valuable resource is E-link. It is only available to students but there are not only job listings, but also alumnae/alumni contacts who WANT to be contacted about working at their particular job and company. It is an invaluable resource.
Summer Internships and job interviews begin early in the fall (mid-September ... see below for specifics).
For those to be seniors, if you think you may want to teach in a public school but do not have the background there is always Teach for America (very competitive to get into, as you are aware). Similar programs are the Woodrow Wilson Fellowship and the Alliance for Catholic Education.
At the bottom of Kris Nairn's webpage is a link to potential career opportunities and websites with information. But if you are interested in a government internship or job (NSA, NASA, Sandia, Lawrence Livermore or Lawrence Berkeley) they have EARLY (typically OCTOBER) application deadlines. Take a look at the websites and get letters of recommendation soon.