Gamestar Mechanic will teach you how to design a video game by playing a video game. Each "Quest" contains missions --challenges where you play, modify and design games to help you level up from player to designer. As you complete the missions in "Addison Joins the League", you earn items that you can use to make your own games.
So who is Addison in "Addison Joins the League"? You are...because Jhansi and Samson's colleague, Rodney, has stolen sprites and vanished into the night - and they need you to be the new designer.
In addition to playing/learning from straight-playing "Addison Joins The League", you can also download a few templates to practice your own game designs. They also offer special challenges where you can earn sprites and other components for ultimately awesome games.
The have a great social media aspect: you can publish your games and connect with a other designers, a community of over 250,000 GM enthusiasts.
"Addison's Complete Quest" is the full introduction to game design and is available for a one-time cost of $19.95. But I think that playing the free version is good enough because it will allow you to learn/understand game design theory...Plus it's fun to earn Sprites!