The one thing you need to know of course is that at the first sign of fruit, pinch it off! Fruit does not reduce like leaves do, and a full size mango on a bonsai would be ridiculous. You might want to consider something more appropriate, like a cherry tree, or cotoneaster (that has small berries), but seeing as your tree has sprouted, just use common sense as you go along.

Thank you. But the question is, how do i make the leaves smaller and the trunk thicker? I dunno how mango trees react to pinching off leaves and mango trees have a fairly large leaf.

You wait til the tree's mature enough to handle leaf reduction, and make sure you do it in late spring, and then just take off e.g. one third of them, leaving the next 2/3rds for following years. That, btw, is my personal advice, and not anything I actually have read or been told... just seems sensible. And if you're asking about thickening trunks, I think you need to do some basic research into bonsai as a whole, because there's really too much for me to go into that here now.

Is the leaves falling off...or another one sprouting up?

I don't know your zone...if these grow naturally in the ground in your location. But, most who try to thicken up trunks if their zone is friendly they sink them in the ground to thicken the trunk. I would not suggest this if your not zone friendly. Leaving a sacrifice branch (one branch to grow longer than the tree one is trying to style also allows this but, you aren't that far along yet.) look into grow boxes...more shallow and wide. Some use plastic colanders or pond baskets as well. But...thinking it might be best to allow it to be spring before messing with it. I'm not familiar with the tree. But, most suggest spring for repots and such.

Defoliation if I am not mistaken...is done to a tree that is practically a finished tree. For it slows down the growth of a tree when done. And that isn't your main goal at this time.

Your tree is too young. wait for a few years