If you're planning on tiling your shower, you'll need to start with a solid foundation. Cement board is the best material to use for this purpose because it's waterproof and won't rot like other types of wood. You can glue shower walls to cement board using construction adhesive or mortar.
Construction adhesive is the easiest option because it doesn't require any mixing. Just apply it to both surfaces and press them together. Make sure you spread a even layer so that the bond is strong. If you're using mortar, mix it according to the instructions on the package and then apply it to both surfaces. Once it's dry, you can tile over top.
Whichever method you choose, make sure the bond is strong before moving on to tiling. This will help ensure that your shower lasts for years without any leaks or problems.