My advice, FWIW, is to go used for your first bike. An older, rigid (no supension) MTB is a great start. Too many people buy a new bike thinking that it's just what they want, only to suffer buyer's remorse a month later. I'm speaking from experience here, BTW. Find a bike and take it to a reputable LBS (local bike shop) and have them tune it up. Slap on some slick tires, fenders, a rack, and some lights and presto... you're a bicycle commuter! Here's an example of what I'm taking about:

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