Not an editor but I hope I can provide some input as an artist since many theories and tips cross mediums.
The first issue with colour can be easily fixed with learning the basics or colour theory. It will help you with matching colours and creating themes/moods. e.g complimentary colours, analogous, etc
Next would be composition. First figure out your focal point and build on top of that. Such as contrasting shapes, colours, and textures to help emphasize your focal point. This can only improve with practice.
Next would be mood/theme (aka the easiest). First figure out the direction you’re going for. Is it calm, scary, cute, happy? Or is it a certain character? And build on colours and composition to fit that “theme.” Font is also useful here. Say if you’re going for a scary look you might go for a font that looks like it’s dripping blood.
The easiest order would be:
theme/picture > colour > texture > details > font
That way you have a sturdy foundation to keep building on your piece.
The one thing I can tell you is that you’re still new and practice and experience are the best way to get better at any artistic medium! Hope this helped (´︶`)