Question
How do I prepare my garden soil for fall gardening?  

Answer
Hi Shari,  I'm going to assume the worst here and imagine this as unimproved ground.
1. If there's undesirable weeds and grass in the area, spray them with glyphosate herbicide.  This is a non- selective herbicide (it kills everything) but is not root absorbable, so we can replant within a few days.
2. Add about an inch of organic matter on top of the dying grass.  You can use grass clippings, leaves, ground bark, composted manures, anything to help hold moisture and help separate the soil.
3. Add lime if needed (have you pH checked at your local garden center).
4. Add a good all purpose fertilizer at label rates.
5. Till all this together into the top 6"-8" of soil.
6. Plant, water and enjoy!
Just a little work and lots of rewards.
Jim Crowder