The most abundantly used nutrient in your garden bed is nitrogen. Microorganisms need it to break down decaying matter and plants need it to make chlorophyll, the food plants "eat" to grow to maturity. Adding nitrogen, if needed, to your garden space before planting should be considered one of the first chores of preparing for the growing season. By understanding how this mineral is used, you will have better results each year.
Cut down into the ground with a clean hand trowel and pull out a three-inch sample of soil. Try to get your sample away from any decaying plant matter like leaves or bark mulch, as these will skew the test results. Package up the sample in a clean container and take to your local agricultural extension office for free testing. They will be able to help you determine the amount of nitrogen and other nutrients needed to return your soil to the correct growing balance.
Purchase fertilizer in the correct nitrogen ratio as determined by your extension office. Most fertilizers are manufactured in a nitrogen to phosphorus to potassium balance. The numbers on the package will show something like 10:10:10, showing the ratio per pound in the packaging.
Spread fertilizer as per the brand's instructions--most of the time by the shovelful first into the soil, turning it over into the dirt about a foot to 18 inches down; and then, working the soil to a powder consistency with the tiller.
Plant your seeds and plants as usual.