Depends on what joystick you're using...
Some joysticks have integrated buttons, which are mapped to "special" values e.g. Grove Thumbstick, others like the Sparkfun/Elecfreaks joystick shield run the buttons to digital inputs which is much easier (can use InterruptPorts) . If you're using a Sparkfun or elecfreaks style shield make sure the voltage is set to 3v3 If you're using an analog joystick and some jumper cables start with A0 & A1. Then add the button to D0/D1
If you want to do a remote control vehicle project (I have robots & Crazyflie nano sampled) check out the elecfreaks style joystick x(4 big buttons, 2 small buttons + joystick push button) you'll need to put a jumper wire on the shield for nRF24L01 wireless to work..
Sample code and applications on my blog