Blood type is a fun and easy way to get to know genetics and learn some practical applications. In the simplest of ways, the three basic blood types are A, B and O.
A good place to brush up on blood type genetics is wikipedia, which has an excellent article on the topic.
Once you’ve reviewed this material (if you need to), then go to nobelprize.org to play a game about blood transfusions (all based on simple blood type genetics).
While you’re on the site, read a little about Alfred Nobel and why it was so important to him to have a lasting positive legacy.