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This is a pretty terrible question. That's like saying "How do I speak [insert language here]?"
It can't be taught on a single whim.
However, I can point you towards some tutorials.
First, check the Tutorial section here. There are some decent tutorials around.
Secondly,
http://www.rpgmaker.net/list-c-rmxp_tutorials.html has some RGSS tutorials by RPG-Advocate.
You could also do well to find the RGSS tutorials written by Astro_Mech, which are very good. I believe they can be found in the RMXP Tutorials section of the forum (see suggestion 1).
Then there is also the Ruby Pragmatic Programmer's Guide,
which can be found here.
There is also
Why's Poignant Guide to Ruby.