The easiest point to jump on is KDE-Plasma, look it up, it is a complete DE that is very customisable and it is all doable through a GUI - themes, widgets etc its all baked in. You can do a lot of customisation in plasma fairly simply, that might even be enough for you, you can really do a lot in plasma and you don’t need to know much besides getting it installed on your system.

From there, if you want to touch something a bit more complicated you could install some window manager to use alongside it and just fiddle with that, look at the configs, understand how that works, try to build your own thing from the ground up, or just copy someones dotfiles here.

E: To clarify, if you use a display manager like SDDM it will let you select the DE or WM on login.