to turn decorative items into useful ones. ie, the scattered pile of books into a 'bookcase', and a decorative backpack into a 'wardrobe'? Is this even possible for a complete noob like me? I'd really appreciate your help!

(Assuming you already know how to make objects in general, or don't mind looking up tutorials to learn how to do so)

The easiest is to clone an object that functions the way you want it.

Removing/adding functions to a deco object is much more difficult/fiddly/tedious/annoying than starting with one that has the functions/animations you want (unless you want something you can't find).

Animations for sims will be the same as the cloned object, so if you clone an ingame wardrobe or bookcase, you'll still get the regular animations of sims taking a book out of a shelf or opening a closet door/drawer. Some CC objects have been edited to remove or tone down these animations, so they work better with a smaller item, so in some cases it can be an idea to clone a CC object that has the options you want (but check if the original creator is fine with it first, and if you upload, make sure to credit).

Other than that, it's much the same as making most other objects (tutorial time).

Just be aware that if you want to keep animations/moving parts or morphs for objects that have those (examples: Morphs - Food states, water in sinks...; animation - Drawers, fridge + oven doors...), joint assignments + comments (Milkshape) need to be transferred to the object, and you also need to import and export the files as the proper format (.simpe or .5GD). It's very similar to making CAS meshes, but usually needs to be done manually. If you don't want animated parts or there's no need for them, it's usually fine with the OBJ format.

Simmer22 to the rescue again!! Thank you so much! I'm really looking forward to trying it out.