I am basically from the world of C language programming, now delving into the world of scripting languages like Ruby and Python.
I am wondering how to do debugging.
At present the steps I follow is, 

I complete a large script,
Comment everything but the portion I
want to check
Execute the script

Though it works, I am not able to debug like how I would do in, say, a VC++ environment or something like that.
My question is, is there any better way of debugging?
Note: I guess it may be a repeated question, if so, please point me to the answer.
Your sequence seems entirely backwards to me.  Here's how I do it:

I write a test for the functionality I want.
I start writing the script, executing bits and verifying test results.
I review what I'd done to document and publish.

Specifically, I execute before I complete.  It's way too late by then.
There are debuggers, of course, but with good tests and good design, I've almost never needed one.
Debugging a scripting language like ruby