Feel free to ignore this advice, all good advice is routinely ignored on the BF: Get a used, like Craigslist/garage sale, rigid mountain bike, probably 15" with the seatpost up, and just ride. Buy some commuter appropriate tires if they don't come on the bike. Over time and with road experience you can refine your preferences and spend more money. The mountain bike will always be worth something to you, even if it's a bad weather bike or grocery getter. Simple.