First come, first serve basically implies that the order of acceptance is just reliant on the order of application. ie if you are the first to apply, you are the first in, second to apply, second in, ect. Rolling basis means that they will send acceptances starting immediately and will continue as applications come in. Highly competitive applicants will probably be sent acceptances right away. Other qualified applicants may sit a little to see how they rank against applications that come in a little later. Sooner is better in that there is a limited number of applicants that can be accepted and the available places shrinks and the theoretical pool of applicants grows over time as more applications are submitted. So submit as soon as you are able to put your best application forward. Hope that is helpful.