Go to bike shops and test ride, test ride, test ride. Ask about bargains specifically. Look for used bike shops (usually found near universities). Even if you buy used, you'll get a better idea of what you like if you've tried a lot of bikes. Bike shopping is fun too! Be patient and you can find a good used bike.
Whether you buy new or used you are probably going to be buying upgrades for commuting
My recommended accessories for a budget:
$80-$100 for flat resistant tires (schwalbe marathon plus, specialized armadillo, or similar). There are no shortcuts when it comes to tires. The cheapest tires I've been happy with are Bontrager hardcase at $35 each, but the armadillos and schwalbe marathon plus are better tires.
$110 for Fenix 2xAA headlight, Eneloop batteries, and Superflash tail light setup
$30ish for fenders if you want to ride in the wet/rain.
$30ish for a cheap rear rack unless you like backpacks. Backpacks are too sweaty as the weather warms imho
$30ish for affordable pannier to go with your rack. Wald baskets
are great as an alternative and will hold most bags you already own (wald baskets are a little heavy but that doesn't matter unless you have a lot of climbs to deal with)
$40 helmet (don't spend more. Go to the LBS and get this)
$30 frame pump
wedge bag loaded with a spare tube, patch kit, Alan wrench set, tire levers, multitool. ~$25-$35
This minimal list is nearly $400. You can skip lights if you never get caught at night, but spending about $20 on "be seen" lights would be wise regardless. The fenix/superflash light setup I listed is a great value. You can skip the fenders if you skip riding in the wet. You can skip the rack and panniers if you don't mind a backpack. You need the good tires, frame pump, helmet, and repair kit ($175 worth of stuff).
Don't burn yourself out at first. 17mi rt is a pretty long commute to ride daily if you are normally sedentary. Set realistic goals for yourself and give yourself plenty of time to get to work on time and "cool down" once you get there.