What you need is a Text class; a class that creates objects that display some texts. A very simple Text class is given here. The code is sufficient to create a text object with some text and also change the text of an already created Text object. As far as the animation with the alien, you need to try to create some code; then, if you have problems with it, post the code you tried, explain what you want and what you do not want (specifically), and ask for help.

thanks danpost. i need further help like on click of mouse i want the object to come near machine and as that object comes near machine a message should be displayed? Secondly, how do i call different class or actors in a real time. As i cannot create an object in a default world.

Again, I would ask that you try to create some (hopefully) workable code to do what you want and if you encounter problems, post the code with an explanation. If you are not creating any code, you are probably not learning anything. If you are at a total loss, then you probably should review the methods that are available in the greenfoot package. If you are not familiar at all with the Java language, then you should probably consult the java tutorials -- especially the section on Learning the Java Language.