You first need to install a shaders mod, although you may already have one.
To check if you have shaders support, and also to enable shaders, check in options and video settings for a "Shaders" option. Choose a shader from the list to use it. If there are no shaders listed, you may have to find some and install them into your game.
Popular and recommended shaders include:
A decent computer is required to use shaders. Many shader packs come with "lite" and "extreme" versions, as well as versions in between, so choose accordingly.
SKCraft