Grab a piece of paper and draw a rough outline of your garden area. If you don’t know the measurements, take a tape measure and measure it. If you are going to build raised beds, decide how large you will make them, and how wide the paths will be. Put note of this on your paper. Figure out your square footage of each crop that you want to grow. To find the space required by tomatoes, peppers, etc. refer to http://downtoearthseeds.com/plantinginfo.php. You may need to adjust your garden area, or how much of what crop you want to grow. Then start drawing (with a pencil) in rows or spaces of each crop.
Put your tall crops such as heirloom corn, tomatoes, climbing beans or peas, on the north end of the garden to avoid shading the shorter ones. Some crops like corn, pollinate better if they are grown in a block rather than one long row. (Corn is wind pollinated.) As you gain experience, from year to year, you will put more thought in your plannning. Consider where you would put a second crop of bush beans. Do you want a path down the middle of 40 foot rows? How will this set up work with your irrigation system? When the onions are harvested, what could you put in their place?
Here is a simple plan for a large flat garden. The smaller crops such as radishes and lettuce and carrots can be put in raised beds or even flower beds close to the house. The options are endless. It all comes down to just doing it. Dig up some ground, or fill up some big pots and start planting. You will gain some wonderful tasting heirloom vegetables to eat and alot of experience!
Most crops benefit from rotation. Do not grow the same crop on the same spot year after year. Do not grow organic crops that are in the same family in the same spot. Broccoli, cabbage, brussel sprouts, cauliflower, and some others are all in the same family. Try to rotate them from year to year. This will avoid some of the common pest and disease problems.