Know what it will take to apply and get into the graduate school of your choice
Are you considering graduate school? If so, begin considering which schools offer programs in your field of interest during the fall of your third (Junior) year. If a summer research or internship program will bolster your application (highly likely in all of the sciences!), start researching these the fall of your third year. The summer between your third and fourth years is the best time for an REU (Research Experience for Undergraduates) or internship. Applications for a summer position are typically due near the beginning of the spring semester. In addition, there may be opportunities to have an undergraduate research assistant position during the school year. These positions are sometimes advertised but are often created on an ad hoc basis, so be sure to talk to faculty about your interests. Spring of your third year is a great time to start preparations for and take the Graduate Record Exam (GRE) if required by your graduate program—you will need to have the GRE scores sent directly to the schools where you plan to apply. International students will need to take the Test Of English as a Foreign Language (TOEFL) exam.
It is also wise to contact potential graduate schools to learn what courses you will be expected to have taken before entering the program. It is possible to take these undergraduate courses after beginning a graduate program, but your application will be much better if you have already mastered prerequisite topics.
After you have completed an internship or summer research program, the fall of your fourth year is the best time to consider graduate schools more carefully and retake the GRE if necessary. Some schools will have applications due even before the end of the fall term, so watch those deadlines carefully!