coin system

Hi,
i have to make a system where guest are allowed to see only first 500 characters of any post and registered users are allowed to see whole posts.In addition, every user would have a certain amount of “coins” any everytime he/she would look a post a single coin would be removed. So, everytime a registerd user reads an article he/she loses one coin.

I have no idea how to tackle this..
thank you in advance

and sory for my bad english