You should attend a few. There are also professional Career Placement companies. These recruiters are generally better at finding jobs for people with a lot of job experience. So, it is not in their best interest to work with new graduates since they don't make a lot to start. These companies usually advertise in big newpapers like The New York Times.
Many campuses hold several job fairs every year.
Some job fairs are targeted to science and technical majors. Your campus Career Development Center should have a schedule of useful events. Job Fairs are also advertised in the local newpapers. Your professors might be able to hook you up with a job interview, or point you in the right direction. Family members and friends might know someone who knows someone who knows someone who works for a scientific company.