I like to buy tomatoes, cabbage, broccoli, cauliflower, brussel sprouts, eggplant, and peppers plants. This year I also bought 1 zucchini.
I plant seeds of green beans, peas, lettuce, spinach, carrots, raddish, and beets and swiss chard.
Squash takes a TON of space, I've never tried it although my parents (who live near Walker) have a huge garden and grow tons of squash every year.
I'd start with some plants and some seeds. I wouldn't add much to your soil. We have sandy soil here too and this is my second year and my garden looked GREAT last year and so far this year we are looking good too.
You'll have to weed. Go out and pull them and remove them completely from your garden space (don't just leave them on the ground, they can re-root). I like to use a small hand held metal rake, but a hoe is a good investment. You'll also appreciate a trowel for digging the holes to plant your plants in.
When in doubt, ask at the garden center for advice on fertilizer. I've never used any.
Also, if you plan to plant broccoli or other cole crops, expect to use a de-bugging powder to keep the little white moths and their babies off. They are nasty, and poop all over your vegi's. It's worth the cost of the powder, and it washes off completely (so it's safe for vegi's).
Good luck!