I'm in East TN (we're already in the mid-70s to low 80s), and I've planted lettuces, radishes, beets, chard, kale, cilantro, spinach, and sweet onions outdoors. For now, stick to cold-hardy plants like radishes, lettuce, beets, kale, etc. If you have a cold snap, cover your containers with a thick layer of straw (this insulates them). For a really great resource that addresses cold-hardy veggies and what temps they can withstand, see: http://www.southernexposure.com/growing-guides/fall-winter-quick-guide.pdf

Lettuce in containers can be started now. Move indoors at night if it drops below 45. Anything frost tender should be started 4 to 6 weeks before your "frost free" date for your area. Don't do them too soon or they get leggy and don't transplant well. For example, here in Chicago, I start tomatoes/peppers around tax time, April 15th. Any sooner and they get too big (our frost free date is May 15)

How do you start a container garden? I would love to do this

We don't do everything in containers, but I do recommend planting lettuces in hanging baskets ... Easy to pick, no worry of slugs etc, and looks nice. Plantation different times so you don't harvest all at the same time.

I live in Wisconsin (zone 5) and planted spinach, chard, salad greens, carrots, and peas on 3-18. We have frost until mid-May so I won't be able to plant tomatoes, peppers, cukes, etc outside until then, but I'll be starting my seeds indoors at the start of April.