The number one thing to look for is a sleeping bag’s temperature rating. This will tell you the coldest external temperature at which the bag can be expected to keep you comfortably warm. Look at a sleeping bag’s length too, especially if you’re tall enough that you might not fit comfortably in smaller sleeping bags.
You may also want to pay attention to a sleeping bag’s weight and packed dimensions, depending on its intended use. These are more important considerations for backcountry campers and backpackers. But if you’re car camping, they’re less likely to be of great concern.