Are you planning to plant a new bed next spring? There are a number of things you can do this fall to get those plants off to a running start:
If you are beginning a brand-new bed, eliminate any grass that is growing there now by mowing it short and covering it with damp newspapers. Add a few inches of mulch or compost on top of the newspapers. In the spring you can remove the newspapers, dig the mulch and decomposed grass into the soil and add your plants.
Spread compost over existing beds. It will protect the soil and add nutrients as it breaks down over the winter.
Instead of spreading compost, you could plant a cover crop such as annual rye grass, which will grow in fall and early spring. A few weeks before you plant, till the rye into the soil. This will restore nutrients to the soil.
Get your soil tested by your local extension service and add recommended amendments now, giving them time to break down and take effect by next spring.