It may not be the first thing someone does after they have started using Adobe Experience Manager, but it is close to the first. Whether to go the ClassicUI Welcome Page instead of the TouchUI Sites Page, or to change what the root of the publish server is, the answer is simple. The top Google hit for “change landing page aem” is the Landing Pages documentation from Adobe. The documentation give a lot of information, but it doesn’t describe how to change which page is your landing page. Jayan Kandathil has a suitably brief description on how to do this.