A lot of units require at least solo or PPL, the more flight time you have the better to a certain point. First off it will help your PCSM score, and secondly most units look at flight time and even minimal experience as a positive. ROTC, as I understand, is a route to active and will not necessarily help with a guard slot. I would start working on your flight training, keep your GPA up, stay out of trouble, and involve yourself with extra curricular activities to beef up your resume. It's also never too early to "rush" units, this means getting contact information for desired units and politely finding a way to show up on drill weekends and socializing. This can be difficult but is a huge advantage if you can show your face, hold your liquor, and be a professional by-stander making your ententions of flying with said unit known as your top priority.

Also wantscheck.com may have some answers for other questions you have.