48x15 will be fine for the first year. Keep it simple and learn the track, the bike, and the technique. Once you have those nailed, then you can start worrying about fine tuning things like that.
Unless your lungs are exploding, and you are consistantly in the top 3 in races - then you can worry about gearing.
First thing on the list: learn how to spin fast and smooth. You need that before you start swapping cogs.