How can I save and read data on Android?

You can use the PlayerPrefs class of Unity to store Ints, Floats and Strings. The documentation for this is at: http://unity3d.com/support/documentation/ScriptReference/PlayerPrefs.html

If you need to store your own data types you can use System.IO classes of the .net (mono) library. The actual paths that you can use are located at Application.persistentDataPath and Application.temporaryCachePath.

You application needs to have permissions in the AndroidManifest.xml to write to storage. That can be set in Player Settings -> Android Tab -> Other Settings -> Configuration -> Force SD-Card permission.