Play around with the other examples in the SDK, making your own changes. Spend time looking through the code trying to understand what it's doing.
Eventually, come up with an idea for a channel and start developing it. That's probably where your fastest learning will take place. One place to start is to look for public RSS (Xml) feeds containing mp4 videos that you can access. For example, the first Google search entry I came up with for "rss feed mp4" was for
http://www.engadget.com/engadgetshow.xml, which contains a feed containing mp4 videos for The Engadget Show. You could make a Roku channel out of that. I don't know if one already exists. It doesn't matter, you could make your own anyway. There are thousands of news feeds, podcast sources, etc. out there that you could take and make into a Roku channel. "mp4 rss podcast feeds" comes up with about a million Google hits, many of them pages that link to lists of podcast feeds.
