There are tons of resources on line. I went through quite an extensive review involving my daughters house, becasue they would get marginal readings...about 3 or 4. After repeated testing, including some expensive ones, and a 6 month test, we decided not to do any remediation for now. The long term test was under 2, and they said that did not indicate anything needed. She opens the windows a lot. We went through quite a lot of fussing, because the high readings were discovered shortly after large new granite countertops installed, but ultimately they were ruled out.
The most important design point is that you must intercept the radon BEFORE it gets in the house. The one thing you do not want to do is ventilate a basement, or otherwise create a negative pressure in the house, which would bring in MORE radon, assuming it is ground sourced.