Howdy Neighbor. OK, peas plant directly into the garden, I normally plant them the last week of April- first week of May. Corn goes directly into the garden one week after the last frost. For me that is 27 May. Tomatoes need to be started indoors. There is a lot of advise already on this site for growing tomatoes so I won't repeat it. Squash you can start indoors but you will want to wait until about 4 weeks before you want to plant them outdoors.
Important things... Seedling need really good lighting to grow well. I know there is a really good thread in the greenhouse catagory about fluorescents. Any make sure your harden off your plants. To do this, place your plants outside in a shady, sheltered area to get acclimated to being outside (I put mine on my front porch). I do this just for a few hours the first day and gradually work up to leaving them outside overnight and in direct sun. My first year I did not do this and lost my entire crop. Luckily because I was jumping the gun on when to start my seedlings I was able to restart a new group and had them in the garden by June.
Good luck.
