Re- iterating comments- Second hand will offer best value for money. Avoid chain store bikes. Much better to get a use quality bike that is 1/2 worn out than a new cheap and nasty one. I suggest a Hybrid (or Comfort) bike with flat handle bars as a good starter. Road bikes are designed light to go fast at the expense of comfort (experienced riders are used to it). A Mountain bike is another alternate that will work, although not quite as well on paved surfaces.
Key things are to get the right sized bike. If at all possible get a cyclist mate to help out (ask around work if you don't know of anyone - I am sure someone will step up).
Visit a couple of bike shops, I recommend find ones with lots of kids bikes as they know about tight budgets and are used to dealing with inexperienced riders. They may have suitable used bikes, or last year model on big discount. Even if you do not plan to buy from them, you may need them fro service, so they should be more than happy to talk about your needs and offer advice.
Don't spend all your money on the bike. You will need helmet, pump, spare tubes and puncture repair kit (learn how to use these). I strongly recommend padded bike shorts. If skin tight lycra is not your thing, you can get baggies (common with MTB'ers) and even ones that are underwear. Really good shorts are expensive, but even cheaper ones will make a difference to comfort when riding.
You don't say what your current fitness level is. Make sure you start out easy, even if you are super fit. Cycling uses different muscles to what you are used to going to hard to quickly usually ends badly. I would say 10 miles a day is too much for the first weeks, but is a good target.
Don't be talked into buying a bike for when you are fitter and training for the Tour de.... If you do you will upgrade it before then anyway, but do by a bike that is of reasonable quality.