Step 1: go to Bikes Direct and order a Motobecane Messenger

Step 2: ride it like you stole it.

Note that step one can be replaced with pretty much any other track/singlespeed bike on Bikes Direct. It's honestly the cheapest way to get up and running. I understand that the parts aren't necessarily the best, but by the time they wear out you'll have a better understanding of what you like/need.