Welcome Sally!
You can buy seeds at most any local nursery, they always have seeds left from the spring inventory. If you don't like the selection you can buy on line. My favorite on line vendor is Baker Creek Heirloom Seeds.
http://www.rareseeds.com/
Another favorite on line veggie seed vendor is Gourmet Seeds.
http://www.gourmetseed.com/
If you are adding to the soil I would suggest you purchase a good quality compost. Look for a local supplier that will sell it by the cubic yard. Places that sell landscape rocks and crushed stone usually carry it.
Easily grown from seed-
Brocoli
Summer squash and zucchini
Winter squashes or pumpkins
Cucumbers
Peas
