Please prepare a micro-SD card and a good quality power adapter and USB cable. Failing to provide sufficient power may cause the system unstable.
Download the OS which suits your need https://www.raspberrypi.org/downloads/ NOOBS: an installer with user interface. good for beginner Raspbian: multi-purpose system you can make anything on Ubuntu / MATE: just like Raspbian, but in different flavour OSMC / LibreELEC: make your PI into a media player