It can seem hard to figure out when to start transplants or sow seeds outdoors. You certainly can't rely on when transplants become available in local garden centers (it's usually WAY too early to put warm season vegetables in the ground).
But the best starting point advice (in the U.S.) is information from our state networks of the Cooperative Extension Service, and their equivalents in other places.
In my state, I can rely on fact sheets from Clemson University (our state's land grant institution, which through its Public Service Activities includes in its mission to provide science-based agricultural information to end-users.) Our CU Home and Garden Information Center's factsheet, Planning a Garden, is a great reference for sowing and planting times in the Piedmont, Midlands, and Coastal Plain of South Carolina. Our neighboring states (and states through the U.S., for that matter) have similar useful information distributed through their equivalent Cooperative Extension Service activities.
Another useful device that I'm using is a downloadable Excel spreadsheet for seed starting times, distributed by YouGrowGirl.com. One of their readers (I think) created an extremely useful chart to determine (based on date of last frost) when to start transplants, sow seeds, and transplant seedlings. Check it out!