The harmonica is considered a light instrument compared to other musical instruments. While for playing other instruments, you normally need a teacher and plenty of practice, the harmonica is the type of instrument that you can learn how to play just by having fun and trying it out.
Many people say that they got the hang of playing the harmonica, just by grabbing a cheap harmonica and starting to blow in it. Of course, if you want to get a little better, you will find plenty of tips and tricks online to help you. And there are many easy to understand books on playing the harmonica.