I will also echo the recommendation for a 26" mtb. Regardless of size, the #1 recommendation around here for 'what bike should I start commuting on' is to shop your local craigslist for a 90's era rigid-fork mountain bike, and put slick tires on it. This will get you steel, so that's good, it will probably have mounts for a rack.
Don't fret yourself about the 5-different varieties of 26"; 99.9% of them out there are good ole 26" mountain bikes.