How do I put shelves in the studs of a wall?

Cut out sheet rock with a knife we’re you want the book case to go when you open the wall make shure there are no electric wires there or you will have to move them so don’t do it near a wall plug or switch.you might need to cut some 2 x4 s away to make it bigger and add them else we’re to open it up more then add pieces of Sheetrock inside for back wall or tape corners and and cover wood with Sheetrock then sand and paint

If this is a newish house the studs will be only 16" apart, and you will have to remove the wall board to expose the studs. Normally in hanging shelves on a wall you would try to have the shelf supports on the studs themselves,

You can buy a stud locator at the home improvement store, then use only where studs are to apply any braces or brackets to hold up your shelves. A trained ear can find a stud by tapping but I would not try it without the device to help you locate them. Ask at your preferred home improvement store for help with finding one. Also, they can recommend what type of shelf you prefer and how to install them. Our home improvement stores ask us how different projects went. They love to help, so befriend them and they will be a great help.