First, determine where you will test the soil and whether you are going to test several patches or just one area. Either way, to test one patch of soil, choose a small area and clear the area of grass and debris. With a small shovel, dig a few inches into the ground, and with a spoon, remove a chunk of the soil. Make sure there are no rocks or insects in the sample. Repeat this procedure with three more holes within a few feet of your first one. Combine all four samples into one Ziploc-style bag. This would be considered one sample of soil. If you would like to check more areas, repeat the procedure with a new bag.
Students can purchase a soil testing kit although some will not be that reliable, and they will not tell students how much of each contaminant is in the soil. If you have time, consider sending the samples off to a soil-testing lab. (See Resources for information on finding labs.) Some labs will have specific instructions for obtaining a sample, so check with the lab to see whether the procedure needs to change at all.
Even if you have stellar research, the project will not do well at a science fair unless you also present it well. Consider purchasing a tri-fold board, and on it place pictures of yourself going through the process of collecting and bagging the soil so that fair-goers can quickly see how you obtained your results. On one side of the tri-fold board, you can also have a bar graph that shows how much of different contaminants -- if any -- were found in the soil. On the table next to the tri-fold, have worksheets that also share your results so that judges and fair-goers can take the information with them.
The specifics of the project will vary, depending on the age of the child. Older students will be able to test and display several bags of soil, but younger elementary-aged children will do better with one sample. The activity can be modified to fit pre-reading children, too. For instance, children can draw pictures of the soil they dug up and the contaminants they might have found once the soil was tested.