Research the hardiness zone where you live. Find out the average first and last days of frost. Don't plant in the garden before last date of frost in the spring and plan on harvesting everything before the first date of frost in the fall.
Select vegetables and fruits that mature in the time period between that last date of frost in spring and first date in fall. That's your growing season. It can be as long as 10 months for areas like Phoenix, Arizona or as short as 75 days for Durango, Colorado. It doesn't make sense to plant a fruit like watermelon which takes 85 days to maturity if you live in Durango.
Start seeds of cool weather vegetables inside six weeks before the last date of frost. Cool weather vegetables include leafy greens, lettuces, broccoli, cabbage and peas. Peas do better planted in the ground rather than transplanted as seedlings.
Harden off seedlings by bringing them outside for longer and longer periods each day before planting in the ground.
Dig the garden area and add soil amendments like compost, peat and mulch. Check with the local agricultural extension of the university near you to see if any other amendments would be necessary. For example clay soils benefit from additional sand and organic matter. Alkaline soils can be offset somewhat by adding gypsum. Soil preparation is one of the most important steps to a bountiful harvest of fruits and vegetables.
Plant warm weather crops like tomatoes, corn, eggplant, pepper, squash and cucumbers when the temperatures reach between 70 to 80 degrees during the day and stay above 60 degrees at night. Watermelons and cantaloupes are warm weather fruits as well. Keep in mind that while the average last date of frost might be May 15, for example, temperatures might not be high enough for warm weather crops until June 15 or later.
Fertilize vegetables and fruits on a regular basis per package directions. Supplement rainfall with watering if need be. Plants should not remain soggy or dry. Either situation stresses the plants. Check the soil by digging down 3 or 4 inches. Don't judge whether watering needs to be done by whether the top of the soil looks dry. Warmer, drier weather means you will need to water more often than when it's cool and humid.
Harvest vegetables and fruits as soon as they're ripe. Don't let the veggies stay on the plants after they're ready because the plant will stop producing new vegetables. The exception are crops that are ready to harvest all at once like corn, determinate tomatoes and onions.