ok, i found the list i compiled of stuff i'm going to grow, other things are accepted, cuz i will probably grow more than this stuff:

beans, corn, cucumbers, water melon, onions, bell peppers, zucchini, cabbage, carrots, eggplant, garlic, okra, hot peppers, potatoes, tomatoes, water melon.

I am not advocating the use of Sq foot gardening, it's just the method I prefer, but in the back of the books Mel Bartholomew has several charts with starting indoor as well as outside.
or as I have done this year, Just read the backs of the packets and sorted out into sow in side 6-4 weeks before last frost date, sow outside 6-4 weeks before last frost date and the smaller pile which is sow after danger of frost, if my memory serves me right, beans, corn and ?watermelon fall into the direct sow after danger of frost, all those left can be started inside except Carrot and Potatoes,
I really don't know when pots should be planted out, as I have, requested my seed potatoes to arrive the week of last expected frost date, but I can't remember whether I was playing safe or acting on advice received :-)

Your zone does not necessarily dictate your last frost date. Check your climate records and figure out when that is, then count backwards.

I am also in Zone 6, and it is generally safe to plant out by May 15. I have pushed this closer to May 1 when the long-range forecast looks good, and I have the ability to protect the transplants. Last year, I started peppers, eggplant and tomatoes on March 1, and with diligent potting-up, had large, bushy tomatoes in gallon pots, and peppers in 16 ounce cups by May 15. This year I started the eggplants and peppers in mid February to give them 10-12 weeks since they take longer to germinate and get going. Didn't get my tomatoes started until yesterday, but the March 1 date worked quite well. For squash and cukes, I wait until April, as they grow quickly, and do not pot up well the way toms do.

Beans - I would sow outside around the last frost date.
Melons - similar to squash, but I'd wait until 2 weeks before last frost, as they prefer lots of heat.
Garlic - I plant this in the fall, it starts growing, overwinters, then gets going quickly in spring... not sure if you'll get much from a spring planting.
Okra - similar to melons - loves the heat.
Corn - inside 2 weeks before last frost, or outside after danger of frost
Cabbage - start it now - same with broccoli, kale, chard, lettuces, cauliflower etc. - they are cool season crops and can handle some frost.
Onions - the nice thing with onions is that you can grow a bunch of seedlings in a cup, and separate them and plant them out. I'd also start those now.