You should start by using some shapes to sketch the body first I usually make the bodies look almost like stick figures
~Here are some examples I found online~
Basically start by drawing something simple like example 1 and then once you're satisfied with the pose go ahead and add some detail for the body like the second picture. (it doesn't need to look as good as the second pic)
Just going into the final detail right off the bat doesn't help at all, you should always start out with a light sketch first to help guide you. Some good artists start by going into the final detail but that's because they've been drawing for a long time and really know their stuff.
Now, for the tree.
Trees don't look like a wide pole with a bush on top, A tree's trunk has detail and isn't straight plus trees have many branches and the leaves don't really cover every branch.
Every tree is different just like every person is different, depending on when it first started growing and the environment it's in will cause it to grow a certain way and genetics may also play a role in it too along with what type of tree it is.
Try to draw at least an hour every day and you'll soon see improvement, don't be afraid to experiment either and drawing tutorials can be vary helpful as well, developing this skill takes a lot of time so don't rush it just enjoy the ride and definitely keep your old drawings just to show yourself how much you've improved, I wish I kept my old drawings to be honest.
Anyway I wish you the best of luck and I hope to read your comic one day!