Knowing the nutrient content and pH of your soil is the first step of getting something to grow in your garden. You may think you're doing all the right things by adding fertilizers or compost, but you may suffer the consequences later for not taking this first important step. You really should know what's going on in your soil before you plant the first seed or transplant. The soil test will either confirm that your growing plots are in peak condition, or you might have to make some adjustments.
There are basically two ways to do this. You can purchase a small kit from a garden center, or you can send a soil sample to your nearest testing location. If you want to attempt to do it yourself, which may not be quite as thorough as the soil service, watch this how-to video.
If you decide to get your soil tested by the professionals, call your area extension office. They work in conjunction with a local university, and will have an address to send it to. Often they will provide a box and application form, and charge a small fee. The steps for taking the soil test will be printed on the materials. This is how you take soil samples for the test. (Note: A soil sample can be collected any time the ground is not frozen.)
When you get the soil test back, this video will help you interpret the results. She is long-winded, but you'll get the idea.
These tests are a simple and inexpensive way to ensure that your garden has the best foundation possible.