Now is a great time to plant seeds of vegetables that like growing in the cooler seasons. These include lettuce, kale, arugula, spinach, radish, carrots, beets, broccoli, cabbage, onions and garlic. You can also plant herbs such as parsley and chives.
You may wish to plant a theme garden, such as a "salad bowl garden". That could include lettuce, arugula, radishes, chives and parsley. This could be planted in a pot so you don't even need to have a garden!
If you're planting seeds, remember that it'll take a while before you can eat from your garden. I plant Renee's Garden "Baby Mesclun Cut and Come Again". It's a seed mixture of various kinds of lettuce. Here is a video that describes planting the lettuce seeds: http://www.reneesgarden.com/articles/video-mesclun1.html.