I second MysteryMeat, the best way to begin is by learning the basics of standard art. However, perhaps you're just looking for some punctual help for your girlfriend's game, and you don't have much time til her birthday. Either way, I'll still try to give the general advice I can give.
Always
look for references when drawing something that "imitates" reality. In this case, when googling "woman lying down back", I found this photo (warning, woman in a bikini incoming) :
What you're interested in here is the curve of her body. Right now your mermaid has zero curves and the shape of her body is inconsistent - when drawing a person, one has to "see" that the person has a bone structure, here it looks a bit like a sock that's been filled with cotton.
I edited your piece with that in mind, I tried to keep it simple. It's not perfect but I tried some improvements :
I did the following :
- Reshaped her body
- shifted the colors (primary colors generally burn the eyes quite a bit, also some contrast between the shades of a color never hurt anyone)
- Tried adding some "texture" impression by dithering the fish tail a bit, and some volume to the hair by shading
I hope this helps a bit !