how to debug unity script?

can i add a breakpoint?

Use the debugger (MonoDevelop only.)

To print to the console:

Debug.Log("hello world");

or to pause the application:

Debug.Break();

Take a look at Unity Script Reference:Debug.

Debugging is very well explained here http://unitygems.com/debugging-game-monodevelop/

Create a text object in the UI of your game and feed it any information you want to see. Best way I’ve found to debug using VS without having the money for the other accessories. #ProgrammerSolutions