If you are going to put shelves on t he wall be careful as it is easy to tear a large hole in the wall. To do this first locate studs, the 2 x 4's in the wall holding the sheet rock up. If you have lath and plaster be even more careful as the wall is way more brittle. Find the studs with a stud finder, it dies not have to be a special one the is one that has a magnet that rotates on a holder. with this find the nails that hold the sheet rock up. The studs will be there. To check if this is where the stud is you can use a little drill bit, easier to mend a small hole, you should hit a hard surface, if you hit empty space then there is no stud. If you have to use the small drill bit to find the stud by drilling a hole every 2 inches or so. @ x 4's are actually 1 1/2 inches wide on the wall.
Once one stud is found there should be another at 16 or 24 inches from that one you first found. Good luck, mending the small hole can be done with spackle or toothpaste. Make sure it has dried before painting. Also if you have to drill a number of holes if you randomly space them vertically but every two inches horizontally it is easier to hide the holes as the will be in different positions rather than all in a straight line on the wall.

Find some wall that won't be seen and practice there first, but honestly it's not very difficult. Borrow or buy a drill, get the right sized drill bits, plastic anchors, screws, brackets and boards. I put some under the big living room windows and the knuckleheads love to sit there and watch kitty TV. Just ask if you have a question but most decent DIY/hardware stores will have somebody to talk you through the project.

Bug probably would love it. You might also be able to find pre-made shelves that you could just hang on the wall with a sturdy bracket.

I think you could do it. Get a handy drill kit. One of those levelers. Your cat would love it.

Its easy lol

Many ways to do it...There are many videos and many different styles...Pick the project that's right for you.