If you are new to riding, you might want to start with a low-end bike from a bike shop. A hybrid would probably be a good idea, and would work fine for your short commute.
I bought a $225 low-end Giant Cypress from a bike shop. I initially had some rear-wheel issues (I was heavier than you), but eventually bought a better wheel.
I later moved to a recumbent, and started commuting on it, then recently upgraded a few things on the Giant, and now use it as my primary commuter.
I just figure it's safer to make sure you are going to ride before spending a lot of money.