Welcome, Bill and congrats on your new bike! I'll give you some general answers and no doubt you'll more from others. Try riding anywhere you feel comfortable: roads, paved bike paths, hard-packed paths. I have ridden my bike with 23c tires (standard racing width) on dirt and gravel hard-packed paths with no problem. You won't want to do mud, sand, rocks, or uneven dirt. Try to move to actual roads as your comfort level increases. You won't be able to go fast enough or far enough otherwise. Carrying stuff. You can add a rack even if your bike doesn't have eyelets on the frame. There are lots of racks available; check your local shop and online resources. If you aren't always carrying stuff, you may opt for a messenger bag or backpack.
Adding drop bars: Yes you can add them later; you will have to change your brake levers/shifters; make sure they are compatible with your other driveline components. This could get expensive; might be better to trade/sell the bike and get a road bike with drop bars. If you find that want to try more hand positions with your straight bars, add some bar-ends. It's a very inexpensive mod which doesn't require changing components. Locking the bike in the car: If you can't get the bike (less wheels) in the trunk, I'd just suggest you cover it with a sheet or blanket, so that they don't know exactly what you've got. If you have to lock it up outside for any amount of time, I'd suggest a good U-lock. Remove front wheel and put the lock through both wheels, frame and fixed object. I'm sure you've seen this. You've got a nice bike; whether it's the right one for you only you can say, after you've ridden it for a while.
