J.L.
Tomato...you can do a bazillion things with them, especially make your own sauce for pasta. mmmmm.
Corn...we used to eat it for dinner....and I mean nothing but white sweet corn.
Strawberries....this was always really hard for me because I wanted to pick them before they were ripe, man they are tastey.
Those are my top three, but I also like carrots, peas, peppers, asparagus, potato, watermelon, cantaloupe, cabbage.
I have no clue where you get good seeds....we lived on a farm and my dad got them bulk (what the entire farm grew we grew in our own gardens).
I can tell you it's wonderful to just go outside in your garden and pick something for dinner. I think I was in my mid 20's before I ever bought corn. Happy gardening! =)