First off, you'll need a thumbnail image (240x180 and .jpg).
Then you need to add a reference to it in the config.txt file. Content Creator Plus (despite being a horrible program) is probably the easiest way to do this for a beginner).
While you're in there, check that you have a suitable description, and set the region to something more appropriate than Tonga...
Once that's done, and the route committed, drag it into the upload bar on the left - the rest is pretty obvious...
Hope this helps.
Paul