The QGIS website will have 2 versions of QGIS available for installation for Window, Mac, and Linux: a long-term release version (the most stable version of the software) and a current version (the newest and richest version of the software). Either version is fine to download.
New versions of QGIS are released regularly, so make sure to occasionally check for new version releases after you have installed QGIS.
See below for videos instructions for how to download and install QGIS 3.x for Mac and Windows 10.
How to install QGIS 3.x - Mac Version
How to install QGIS 3.x - Windows 10 Version