If you're starting seeds indoors, March is the time to get started. Seeds often need several weeks to sprout and get strong enough to put outdoors. The seed packet will usually give instructions for starting seeds and when they can be planted.
Here's some inspiration and how-to's from the Kitchn:
How to Start a Green Roof Garden
How to Start a Community Garden
How to Make a One-Pot Herb Garden
Planning a Window Box
Victory Garden Project
Our Small San Francisco Container Garden
Book Review: Designing the New Kitchen Garden
And a few more resources from around the web:
The Old Farmer's Almanac - Handy for predicting when the last frost in your area will be
The National Gardening Association - A good all-around resource for how-to's, pest control info, and buying guides.
Starting Seeds Indoors from Garden Guides
How to Construct a Raised Planting Bed from WikiHow
Vegetable Gardening in Containers from AgriLife/Texas A&M
What are your garden plans this year?
Related: How Far Does Your Food Travel?