Your foundation type shapes both where you test and how radon gets fixed. Here's what the EPA says.
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The EPA says to test the lowest lived-in level of the home, for example the basement if it is frequently used, otherwise the first floor, in a regularly used room but not a kitchen or bathroom (EPA Citizen's Guide). SD DANR gives the same lowest-livable-level rule (SD DANR). Radon enters through pathways such as foundation cracks, utility penetrations, sumps and floor drains.
According to the EPA's Consumer's Guide to Radon Reduction, these homes are usually fixed with one of four kinds of soil suction:
The EPA says submembrane suction, a high-density plastic sheet over the earth floor with a vent pipe and fan drawing radon from beneath it, is the most effective method when properly applied. Active crawlspace depressurization is a less favorable option that generally doesn't work as well and needs attention to combustion appliance backdrafting. Crawlspace ventilation can sometimes help, but in cold climates pipes and appliances there may need protection from freezing (EPA).
Many homes mix foundation types, like a basement under part of the house and a crawlspace under the rest, which may need a combination of techniques. A local pro can inspect and quote your home: use the form above.
The EPA says basement and slab-on-grade homes are usually fixed with soil suction: subslab, drain-tile, sump-hole or block-wall suction. Active subslab suction is the most common and usually the most reliable method.
The EPA says submembrane suction, covering the earth floor with a high-density plastic sheet and drawing radon from under it with a vent pipe and fan, is the most effective way when properly applied.
The EPA says to test the lowest lived-in level, for example the basement if it is frequently used, otherwise the first floor.