To start playing with the raspberry pi you will need an OS... and my first choice is always the closest one to Debian ;).
Go ahead, download raspbian via torrent and remember to check the file integrity with SHA-256
We are going to unzip the file and copy the content into the memory card (change of=<memory_path>), dd is a powerful so please be careful, you can lost your data if the path is wrong.
unzip 2017-08-16-raspbian-stretch-lite.zip dd bs=4M if=2017-08-16-raspbian-stretch-lite.img of=/dev/mmcblk0 conv=fsync
Before we put the memory card into raspberry pi, it's useful to enable ssh. To do this you need to mount and create an empty file on the SD card named ssh inside boot directory
Now you are ready!, insert the memory card into your rasberry pi, start it and try to log in via ssh. Most of the cases your ip will be set by dhcp server. The first task is always to change the default password
Start building something with your localhost server :D