Make a list of colleges or universities you may want to apply to next year. In-state institutions will offer lower tuition rates and more attractive scholarship opportunities, but don't let that stop you from looking elsewhere. Browse the websites of the schools on your list, research what majors are offered, and narrow down your selection accordingly. Look for the 'Academic' section, or one similar, to find a description of available areas of study.
For instance, on the undergraduate admissions page of Penn State's website, I find this list of majors. I notice that the college of agricultural sciences offers a horticulture major, and gives students the option to specialize in floriculture. The college of science offers a biology major with the option to emphasize plant biology. I choose Penn State as my example because it has a volleyball team, but I agree with UDGags--don't let this become a make-or-break factor in your application process.
I dual-enrolled at a local community college during my senior year of high school. My advice is to knock out as many general education requirements as you can. We're talking general social science classes, like psychology and sociology; introductory science classes, such as chemistry and physics; basic mathematics and writing courses, your college algebras and English 101s. These pesky courses are required by most degrees and will plague your schedule during your first few years of higher education. Use the aforementioned websites to determine which are most likely to be required for your field of interest and knock them out early. They are often easier at the community college level.
Course codes are often helpful in determining which credits will transfer from one college to another, but this is not always the case. I notice that Introductory Sociology has a different course code at Messiah (SOAN 101) than at Penn State (SOC 001). Check in the 'Transfer Students' section to confirm which incoming credits will apply, or contact the institution's admissions office directly.
I find websites such as ratemyprofessors useful when enrolling for classes.
Don't let my input ruin dual enrollment--it can be a lot of fun. If you're taking, for example, three classes at Messiah next semester, allocate two to satisfy general education and pick a fun or interesting third class (I chose Creative Writing one semester). Just bear in mind that arriving at university with most of these requirements behind you will free up space in your schedule for French, photography, and volleyball, in addition to new interests you're likely to discover along the way. Also note that if you acquire a certain number of credits (in Florida, 60) you will need to apply to the next college as a transfer student. Good luck, orchidman!