You'll need to find your Pi on your network if its headless and you don't know it's IP. I used nmap. On MacOS this is available via homebrew:
$ brew install nmap
#Find your Raspberry Pi
To use it, from the command line, tell it to scan your network for machines with port 22 open (the SSH port):
$ nmap -p 22 --open -sV 192.168.0.* -Pn