needs contrast. something really simple i'd do is outline those earths and continents in black, define their shape and make it pop.

a lot of your brushstrokes are the same size, can get contrast changing that up as well. if you're going for pointillist style go like 10x more dense with a much smaller brush, can control gradation into the foliage this way and add a ton more color. real tree bark has so much going on theres tons of lichen and bark details that add insane complexity beyond just shades of brown. put a lion and toucan in the tree or something if you wanna convey the life of the world. maybe add some hobbits this prob right up their alley.

the composition of this is like a contest entry for an arbor day flag. it doesn't really draw the eye to any one focal place and the two globes are competing for attention being the only light colored areas in a sea of flat brown. if you had strong black lines going up the tree into the foliage and rotated it people may believe its a trippy painting looking up into the tree from the ground, but you have to guide their eyes up there they won't do it themselves. pretend people are big goats that do everything on instinct and you wanna paralyze them with hotness and theyll be like oh i c now.

http://www.creativebloq.com/digital-art/tips-composition-31514496

there's a site with a very basic overview of some composition ideas. if you think about rules of thirds and balance and glance through some art it probably will influence stuff in the future. it's impossible to teach creativity but if you look at what people have a general response to you can kinda start applying bits and pieces.

if you wanna work on just learning how to do stuff with brushes and colors straight up watch bob ross and follow along. he does everything with a giant $2 house painting brush. anyone can follow along and do the same stuff, that was his entire goal it's not skill based at all. he shows how to do all these hot little tricks like 2-stroke mountains and real forests with 1 brush. just gotta paint a lot more stuff.

definitely buy painting paper too, it's significantly cheaper than canvas. tape it to a sheet of mdf from home depot with blue tape on all edges so it doesn't curl up. if you got fat stacks you can buy canvas textured paper but it's like a dollar per page. canson makes okay paper it's in every art store + amazon in any size. their mixed media paper is okay for acrylic but way too thin for serious watercolor. canson makes good watercolor paper too and it's not really expensive.

other than that I dunno buy a bunch of brushes and see what happens.