How to enable SSH on a Raspberry Pi, without monitor:
1. Plug the Raspberry Pi SD-card into a SD-reader and connect it to a PC.
2. In the root-folder of the SD, create a file named "ssh" (without extention).
3. Plug it back in the Raspberry Pi and turn it on. SSH will start automatically.

Pro tip: If you don't know the IP address, use its hostname. Example:
ping raspberrypi.local