Well.
Quote: " WHERE DO I BEGIN???" "
first off you are going to need to learn to program in C++ and the DGDK. I am not being smart. i mean , learn all you can about programming from the very start.
Fundamentals first. ALWAYS.
while your doing your computer science classes learn all you can about the way a CPU works and how the ram works and learn how it all relates to your programming.
While learning that You can start designing your game
do up a Design document. work through your game idea from top to bottom (story , Characters, items, Environments) and how you want it all to be set out.
now you should be aware that a game needs art and unless you are an artist..of Godlike abilities, You cant do all the art and programming you need and create a game in a reasonable time frame.
so at some later date you are going to need other people working on it with you. but you don't have to think about that now.
(sorry if i have ranted here and haven't given you the answers you wanted..)
good luck with your studies and the game