How to start a cartoon?

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So recently I decided I wanted to start writing, drawing, everythinging my own online cartoon. I think it would be like the greatest thing ever. Well I don’t exactly know what to do. Like I mean I know how to draw and scan stuff in and color it and whatever but like… do you add speach bubbles over a finished room thingy or if you draw them in with the pic how do you really know how big to make them? How do you do the squares and stuff… just crop stuff to fit I guess? See I’m really lost as to how to do it all so if anyone has some good pointers and/or tutorials that would be awesome.