Instructions
Find a resource pack you like. There are numerous collection sites around the Web to choose from, as well as communities that are useful resources for creating your own.
Copy the pack, still in its ZIP archive, to the "resourcepacks" folder in your "Minecraft" appdata directory. On Windows PCs you can find that by entering "%appdata%" in your Run dialog. On a Mac it is located under Application Data in your Library.
Run "Minecraft" and open the Options menu.
Click the "Resource Packs" button and select the one you want to add in. Every ZIP archive you add will be automatically placed in this list.