Thanks for liking my work!
Well, i sadly don't use medibang myself...
I can recommend doing real life color studies or photo study paintings for sure.
Mainly to get a feeling for your brushes and how to get the paint flowing. Of course you can also study the light etc. but first you need to find a technique that works for you, i think.
Sorry i can't be of more help. My own technique is rather basic:
Lineart - paint under the lineart - flatten it all and paint over everything in the end.