Fall gardens can still produce a lot of great stuff! I'm assuming you've already picked out the veggies you will be planting? Because of the heat outside, you will actually want to try to start some plants inside so they aren't too stressed. When they get transplanted depends on how hot it is and how long your Fall season tends to be. Just check each variety you plant for recommendations on when they should go in the field. For example, beets should be direct seeded, and can begin again once nighttime temperatures begin cooling off. Be sure you leave about 1 month before your first expected frost, from you last seeding.
Right now is a great time to check the nutrients of your soil (ask a local extension agent how to mail in samples) to make sure plants will be nice and happy, then add whatever you need for optimal nutrient levels.