Hi Daniel,
I've been to the islands several times now so can maybe offer some helpful tips...
In August expect it to be hot and humid, but take layers as it can still rain and get chilly.
Cheri is right that staying in Ponta Delgada is your best bet. The second most common option is probably the pricier spa/resort town of Furnas, up in the mountains. Consider that AirBnB and their ilk are wreaking havoc on the housing market there (and in the rest of Portugal). Foreigners and investment corporations are gobbling up residential property for short-term vacation rentals, thus reducing the supply of long-term housing and driving up prices so locals can't afford to live there anymore. So I suggest avoiding that if possible, or if you do use it, try to make sure it's an actual local owner hosting you. I've had some really good experiences with the latter.
You will not need a car in Ponta Delgada, in fact it's a hassle to have one (parking, tiny one-way streets, traffic). Getting around on foot is easy and more enjoyable and you'll see more. There are buses too, and I know there is (or used to be) a really cheap one from the airport that drops you downtown. For seeing the rest of the island, you can certainly hire a taxi guide as Cheri said, and they will usually charge by the half day or full day. You might want to do that once to get a feel for the place, but I highly recommend renting a car to get around the island. A guide will only take you to the most obvious places, and I found it's easier to relax and enjoy the places you visit without a guide wanting to hurry along to the next stop on their checklist. Rather than rent a car for the entire time you are there, consider giving yourself a few car-free days in Ponta Delgada to get acclimated, and then rent a car by the day when you want to get out and see the sights. You should probably try to book a rental car sooner rather than later, as things get busy in the summer (prices will be high in August). But I've also had good luck with people recommending folks to me outside of the big rental companies, such as mechanics, who rent cars as a sideline. Hotels and good AirBnB hosts can help with this.
In terms of driving, they use the same side of the road as in the US. Main things to remember are to never drive in the left lane on the highway unless you are passing (they get very cranky about this), and (I think) you can't turn right on a red light. And be prepared for intersections with roundabouts rather than stop lights. The highways are good, and it's hard to get lost - there is basically one main highway that goes around the perimeter of the island (with an older, smaller ring road running between that and the ocean that goes through the towns - slower, but you'll get more of the local vibe), and a couple of major roads that go across. And even if you get lost, you won't be anywhere too remote and you might stumble upon some sweet place.
Good luck!
Steve