Start investigating summer opportunities early - the previous fall or winter. The best positions are usually filled by late spring. The best piece of advice, regardless of your interests or which school you attend, is get to know the faculty. If you have questions about a course, ask your teacher after class or during office hours. If you are interested in laboratory or field work, let the faculty know and offer your services.
Most plant scientists are very willing to talk about their careers.
Schedule appointments to visit with several botanists at a nearby college or university. Ask them about the nature of their positions and about the specific interests they have in plant biology. Internships to Explore Employers are looking for experience and development of skills from that experience. Your first and second years of college are not too early to think about getting an internship. If you do not have the time to make an internship commitment, think about volunteering several hours a week on a regular basis.