Here is how I do it.
1. Take a seed inventory
2. Make a "to buy" list. If you are really organized, you have kept a running list all year long as you ran out of seeds. If you are me, you kept a so-so list and step 1 assures you don't get to the first week of April and not have seeds for something you really love to grow.
3. Draw out your garden and start to plot what goes where. Consider crop rotation, space needs, compatibles vs not.
You might ask, "isn't there an app for that?" There is a really good computer program that you can get over at Kitchen Gardeners international.
I like to do it like this though
4. Get input form your family. It only helps to grow 5 kinds of beets if you someone else is going to help you eat them. Besides, if you have little kids like I do, it gets them in on the action.
5. Get out those beautiful color seed catalogs that have been coming in the mail and start shopping. My current favorites are Seed Savers Exchange, High Mowing Seeds, John Scheepers Kitchen Garden Seeds, and Adaptive Seeds.
Happy New Year and New Garden!