Navigate to 'media' on the left-hand menu. From here select 'add folder' which will allow you to keep your media organised. The 3 dots on the side of your folder will provide you with some folder options such as rename, sub folders and delete.
Make sure you have your folder selected and then you can add your content.
There are two options for uploading; 'add assets' and select the file type you would like to upload. This will allow you to browse to select the media. You can also drag and drop the files you would like to upload and they will process to the folder you have selected.
It is recommended that the content you are uploading matches the resolution of your media player or screen.