How Do I Become A Stand Up Comedian?

This is going to sound a little simple, but, write some jokes and then stand up in front of people.  You need to be particularly funny to other people not just yourself, despite the vast possibilities for the range of your act, from standard jokes to stories to the way you do your stand up, your act needs to be unique from the next.

You need to be funny.  No, really.
You need to be REALLY funny.
You need to make people laugh so hard it hurts.

Your material must make us laugh without resorting to racism or sexism these days.

The best material makes us laugh in understanding or recognition.

Next you next to find a venue that will let you stand up in front of people, then try out your jokes.  It's up there that you will find out what makes a good and a bad stand up.

Is there such a thing as a sit down comedian? I wonder how the word 'Stand Up Comedian' evolved?

What you have to do is go to places or restaurant that has a stand up. Then decide for your self if you can do better than those that performed.
Then talk to the owner of the restaurant or place and tell the owner/ manager that you are looking for place to perform, and ask for a schedule when you can audition.
Make sure you have something ready just in case you are told to give an example . Maybe about 3.
Then wait for the decision - do it over and over until you found a job doing a comedian Stan up. Good luck. Humorous one.

U can stand up as a comedian when you seriously make fun :) in a natural way.