Starting out I'd suggest using instance variables.
Click the object add a variable in preferences to the left.
You can then click different instances in the editor to change the instances variable, or set the variable when the instance is created.
To pick the specific instance you use the compare instance variable in the conditions of the object.
Going forward there many many different ways to pick instances, with things like frame, opacity, and even using the system to "pick nth".
Do the getting started tuts. Seriously.