Hello,
          I have heard that people uses whiteboard software on which
one can write or draw using digital pen for e-learning, and I have
tried to use one such thing and felt extremely difficult for me to
write even a single letter. My doubt is that, is this a common problem
for every one who first uses it? By experience can one learn to write
using digital pen? What is the usual method to do so? How long will it
take to master the art?

Are you talking about a product which lets you write on a whiteboard
and then transmits what you write to a computer?

We use this product at work:
http://www.e-beam.com/products/whiteboard.html

It works pretty much as advertised...you write on the whiteboard and
it transmits  whatever you write to the computer.  No issues at all.

So, if you are using a product like this and you're having trouble,
I'd say the product is either not setup correctly, is of poor quality,
or is defective.

yes feldersoft is right we use an active whiteboard in our school and
although we rrely get to use it the first time i tried it worked ok so
if you cant master it it is probably a fault in the hardware or
software somewhere