- You don't really need a minor, but a minor could be selected based upon what you want to use the computer science for. For example, if you are interested in bioinformatics, you might minor in biology. If you were interested in computational neuroscience, you might minor in neuroscience or psychology. Math is another good minor in general, or statistics if Stony Brook has that.
- You ask a professor something just like you ask any other human being :) First, you establish a connection in some way. Take a class or approach that professor during their office hours. A polite email works, too.
- Your GPA and research are very important; GRE is less so, but necessary. Internships might be important for professional programs and jobs but not necessarily for grad school. Keep your GPA up. After you have taken some introductory CS classes, you can explore the potential to do some research in CS.
- Usually undergrads assist a professor in their research in their lab. You need some intro classes first to be useful in the lab, which is why most people start their sophomore year at the earliest, but after your freshman year you can start to take note of which professors in your department have research that seems interesting to you and investigate the possibility of volunteering in their lab. Some professors have formal processes (like an application) and others have less formal processes (you just ask). When the time is right you can find out by sending a polite email or approaching them during office hours and asking.
- Oh god, please don't start studying for the GRE yet. Way too early. You don't know enough to prepare for the CS GRE anyway, and there's no need to prepare for the general GRE. Wait until your junior year.
My advice: Stop worrying about graduate school and enjoy your freshman year of college! The best thing you can do right now is do well in your intro and GE classes and start to explore your interests a bit. There is plenty of time to worry about grad school later. (And there’s pretty much nothing you can do between now and the beginning of college to prepare for graduate school, so relax! Stony Brook is a great school.)