This guide assumes that you've already installed Raspbian on your Pi. If not, go ahead and install it. Then put the SD card into your computer.
You'll have to locate the boot directory, on my Mac it's in /Volumes/boot.
If you're on a Mac, open Terminal (Applications > Utilities > Terminal) and type:
On Windows, you'll need to use Command Prompt instead.
All you have to do is create an empty file called ssh.
If this file exists, ssh will be enabled when the pi is booted.
Connect the Pi to a power source to power it on. As long as the Pi is connected via an Ethernet cable (or wifi), you should now be able to ssh in to the Pi.
If you don't have easy access to an ethernet connection, you can configure a WiFi connection similar to how we enabled SSH.