How to become comic strip artist?
Just do it, develop your own style.
You can check out other peoples work and develop your own style from that.
But you will have to develop your own style and not stick in copying others.
When looking at published cartoons you will see a very wide range of styles but also a lot of cartoons that are not well drawn, just enough of a drawing style combined with 'story lines' or 'insight' that makes you come back to the comic every time.

If you are over 13 and not making comic strips all the time, I doubt you will ever be a professional.  When you are 16 or over, send samples of your work to all places that publish comic strips and see if you can get them to answer you.   

On the other hand, if you just want to do it for your own fun, you can use all kinds of books or online drawing teaching.

I was going to google a site for you but the search did show up so many results that you will need to do the search yourself so you can select those that attract you or those that you feel are styles you can master.

Start a search with 'drawing comic' and follow one of the sugestions the search sites offers. I followed 'drawing comic book style' 
You will get a series of options in videos, in sites that teach you 'all you need to know' as well as sites that combine further options.