An important part of the BGE is the BGE message system. It is very easy to use. But it can be very confusing and error prone if the user does not really understand it.
Therefore Prof. Monster presents:
The BGE Guide to Messages
It will help you to get an inside on the messages when to use and when to avoid. It includes a detailed example to implement a health bar as text and as bar.
The demo file is made in BGE 2.62.
I hope you like it.
Feedback is always welcome
If you think you need some practice you can have a look at the exercise.