Q: Where do I download GeoServer?

Go to the download page. There are generally three options: stable, latest, and experimental. If you want to play it safe go with stable. If you want the latest features and bug fixes, go for latest. Experimental is specific coming features released from branches. To get GeoServer running quickly, look no further than the Quickstart. It is a good idea to read the User's Guide when you are are just learning GeoServer.