Many years ago, right up there with the excitement of looking through the Sears catalog each December and dreaming of magical Christmas presents, was the excitement of getting seed catalogs in the mail. It may have been the dead of winter but just the idea of all of the amazing things that could be planted in the spring, brought to life by the colorful seed catalogs, was a level of fun and excitement that would be hard for people of this generation to understand ("you mean you write to a company and weeks later you get a catalog in the mail then you mail in an order with a check and wait more weeks for your order to arrive???). While times may have changed as far as the method of ordering from seed catalogs, December is still the time to get started on next year's spring and summer garden.
First, you take out your garden journal and review what grew well last season and what didn't do so well. Did you have too many of one kind of plant and not enough of other plants? You also need to diagram how your garden will be laid out in the spring (crop rotation is still a thing even in a small garden). Will you make raised garden beds? Will you do "square foot gardening"? Will you try no-till gardening?
Then you need to decide what you would like to plant. Fortunately you no longer have to wait for seed catalogs to arrive in the mail (although most companies still have catalogs) to determine what you would like to have in your garden as many seed companies now allow you to shop online. Heirloom vegetables are quite popular, especially among gardeners who like to save their own seeds. Some people like to start simple (herbs are really easy to grow) and for long term planning people will often start by getting perennials as well as fruit and nut trees into the garden as quickly as possible.
The best advice is to start small (ordering 50 tiny packets of seeds equates to a full time job when you realize how much work is required to start, harden off, replant, harvest, and process all of this produce!). Always ask for help if you need it (there are YouTube videos on everything gardening related) and utilize your county extension office and Master Gardeners if needed.
Most importantly, gardening is about being creative (grand dad never liked to buy things when he could make them himself--he probably had the fanciest bean trellis in the county which he welded himself out of scrap iron) and experimenting (he tried all kinds of crops, some which surprisingly did well and many others which failed, one year he wanted to keep the plants warm so filled up old milk jugs with water to capture heat from the sun to keep the plants warm overnight--that was a one-season experiment, and we never grew the "largest pumpkin" at the county fair but we certainly gave it a try).
Even if you only have a small deck, growing a couple tomato and basil plants in a container is a great first foray into gardening.