Another way (which I usually use) is to plug in a USB ethernet adapter (or on Pis with LAN, just plug in onboard), then when the Pi is booted, use something like Fing[1] to ID the new IP address on the network. SSH into it (it's enabled by default) with `ssh pi@[ip]` and away you go!

Look also at the official Raspberry Pi documentation[2], which is more succinctly summed up on SO[3].

For WiFi, you can configure the microSD card in another Pi (I usually mint cards on my Mac, then configure a master copy in a Pi 2, then drop it into other Pis headless) so it has the SSID/PSK in the wpa config file.

[1] http://www.overlooksoft.com/fing [2] https://www.raspberrypi.org/documentation/troubleshooting/ha... [3] http://raspberrypi.stackexchange.com/a/13937/6506