Plan Your Garden
Information on the back of seed packets or tags will let you know how far apart each of your plants should be. This is a good place to start if you're not sure how much room each plant will take up. If you think the plant you have will grow to be long (vines like zucchini or cucumber or squash) then you can also plan for them to go vertically in your garden.
Right is an example of a simple square foot garden.
Know What Plants Get Along
Plants have friends and enemies just like people! Getting to know good companions can save you a lot of grief, and can significantly increase production.
Below is a companion planting chart to help you pick good neighbours.