Here is Einstein's original paper on the topic, which is actually pretty readable:
http://fourmilab.ch/etexts/einstein/E_mc2/www/
Of course there's no particular reason that we have to go about things exactly the same way as Einstein. There are various ways of getting this result.
In general, you're not well advised to try to learn relativity from scratch simply by posting questions on this forum. You need a book to present things in an organized way. Some good books about special relativity are (from easiest to hardest):
Takeuchi, An Illustrated Guide to Relativity
Mermin, It's About Time: Understanding Einstein's Relativity
Taylor and Wheeler, Spacetime Physics
The advantage of Takeuchi and Mermin is that they require very little background in math or physics. Their disadvantage is a total lack of connection to experiment.