I don't really know how to help you, but I think it's interesting to start with your "own aesthetic" (example: cover in blue tones with flakes, pastel pink with hearts, black and white with manga panels). It's good to have this base before starting, so that all the decoration fits together as you do it.
When you see that you've already decided on the palette and general aesthetic that you like best, there are a few options: watch some tutorials (there are some very good ones on YouTube, but you can find them on Wattpad) and do it yourself or ask someone to do it. (there are books on Wattpad specifically for this type of order, some people do it completely free of charge).
Three very good apps for working with this kind of thing are: PicsArt, Ibis paint X and Pinterest.
Pinterest: if you use the words correctly, you can get excellent materials, including links to resources for editing (example: 'overlay', 'fonts').
Ibis Paint X: It's literally a board for you to use however you want, but it has a LOT of tools and takes a while to get used to. In it you can make everything from those shiny titles to coloring manga panels easily.
PicsArt: This is elite, you can do everything. I have no words to describe it.
I generally use these three, and the results are very satisfactory. It's not the best instruction in the world, but I hope it helps you! If you need help, you can message me ( ◜‿◝ )♡
(sorry for the English, I had to use the translator ( ;∀;))