Of course, the main data access is the USGS Earth Explorer which is a web application allowing for browsing and downloading Landsat images (and images acquired from other NASA satellites).
It is a helpful guide for downloading Landsat data, describing clearly the required steps: from log in, to searching and selecting the datasets. Each step is illustrated with screenshots of the web application, making this guide very easy to follow.
Please, remember that a Facebook group and a Google+ Community are available for sharing information and asking for help about the Semi-Automatic Classification Plugin.