To install ROS Indigo, visit this link. It will guide the installation process step by step.
There are a few items of note regarding these instructions. First of all, beware that the installation process will take 8 or more hours to complete. The installation process will involve compiling many libraries and packages from source, which takes a long time on the Raspberry Pi's hardware. Secondly, during the installation process you will likely encounter errors as you go. Ignore these errors and continue on. When you are finished with all of the steps you will be able to run roscore and other applications.