The Joystick Shield kit by Sparkfun has all the parts you need to enable your Arduino with a joystick! The shield sits on top of your Arduino and turns it into a simple controller. Five momentary push buttons (4+ joystick select button) and a two-axis thumb joystick give your Arduino functionality on the level of old Nintendo controllers. Soldering is required, but it's relatively easy and requires minimal tools. Sparkfun even have a step by step guide!

The momentary push buttons are connected to Arduino digital pins 2-6; when pressed they will pull the pin low (utilizing the internal pull-ups of the Arduino). Vertical movement of the joystick will produce a proportional analog voltage on analog pin 0, likewise, horizontal movement of the joystick can be tracked on analog pin 1.