Do you have a city in Georgia?
How tall are you?
Mechanical Aptitude?
Target Budget? $100, $300, $500, $800, $1000, $2000?

Where do you plan to store the bike when you get to school?

There are often some good deals on used bikes on Craigslist. But, a lot of bad deals too. I consider most bikes on Craigslist as needing a full tune-up, even if the seller has claimed that it has recently been tuned.

15 miles is a pretty decent commute, but certainly not impossible. Personally I would go with a drop bar road bike.

Don't over-spend. College campuses are notorious for thieves. Plus, if it gets a lot of rain, it will see some wear, and reasonably narrow tires (less than 28mm).

Many people will recommend checking out a few local bike shops. I would not go to a shop only to buy the same thing (or similar) elsewhere or online. But, you may make a good working relationship with a local shop.

BikesDirect has a lot of mediocre quality bikes, but decent prices, I think. Something to drool over.