Welcome back, Saac! 
I think you did the colouring pretty well, considering the character is dressed in grey and black. Advanced artists often use colourful light and (opposite hue) shadows to make the picture more interesting. For example warm reddish light - cold blueish shadow. This way there is some colour even on a grey shirt.
In this post, I have gathered some tutorials, also for colouring/shading:
As for the background part, the first step is to have an idea for the background! You can approach in slow steps if you want:
Single colour background, see how it interacts with your char.
Splash/box/circles of random colour.
2D scene, like simplified landscape.
Complex 3D scene drawn with perspective tool.
Multiple side characters in that scene…
So many ideas, but we can’t help if we don’t know what you like to draw and how much time you want to invest. 
Asking for advice and studying tons of tutorials before even attempting may be a prime way for procrastination by the way. Don’t ask how I know. 