Let us start from the assumption that you now have a hand-held Geiger counter and you want to make readings. Here are some tips:
Background Level
See previous post about background levels of natural radiation. You can use the numbers from that post but it would be better to go online and see if you can find background radiation measurements from your state, county, municipality or a university in your area. A little research should turn up a good background number for where you live.
Atmospheric Measurements
Measuring radioactivity in the air will depend on atmospheric conditions. There will be detailed online weather information for just about anywhere in the United States that is near a town or city. Learn about prevailing wind directions and speeds where you live. Also track humidity levels and air pressure. If you are doing readings of atmospheric radiation and detect levels substantially above normal background levels, be sure to note wind direction, speed, humidity and barometric pressure that are associated with that particular reading.
Measuring objects
We live in an interconnected global world. Goods are being shipped around constantly. You may be receiving parcels from anywhere or purchasing items from any continent. If you are testing items that you have ordered or purchased, hold the detector as close as possible to the object and then move it around because radioactive emissions may be asymmetrical and you may get a different reading from different orientations.
Measuring food
In a study of what are called food miles, it has been found that some of the items on the average dinner plate in the United States have traveled over 1500 miles. In some cases, food has traveled much further than that. Fish pulled out of the ocean off the coast of California may have come all the way across the Pacific from Japan. Radioactivity in food tends to accumulate as it is grown or grows. Then it can accumulate in your body over time as you eat more of something. Since there is a growing debate as to whether there is any "safe level" of radiation even low levels in food might be a health issue. Handheld Geiger counters are not sensitive enough to pick up very low levels of contamination but will detect serious levels of contamination that will mean the food is not safe to eat.
If you are going to test food and you live in a brick, stone or concrete building it would be best to go outside to do the testing. Best to test on a wooden table because stone, tile, concrete or brick surfaces have higher background levels. Best time to test is when the background radiation is lowest which is just before local sunrise. Get as close to the sample as you can but don't touch it because you might contaminate the Geiger counter. Use at least a pound of solid food or a quart of liquid for the test. The longer you test, the more accurate your results will be.
Logging your tests
Keep a log book for your tests with columns for things like time, date, place, item, reading, As you accumulate data, you will be able to analyze readings over time to see if patterns emerge.