This article will use one digital input pin (PortA pin 3) to readout state of push-button connected to it. Pull-up resistor must be connected to this pin to differentiate voltage on pin when push-button is pressed and when it is released. In short period of time when push-button is pressed, voltage on input pin connected to is very noisy and this can make problems. This is shown on picture on left side. Microcontroller would interpret this as multiple push-button pressing and releasing. To eliminate this we use debouncing.

Here is example that is complete project with step by step guide that will show you how to use GPIO pins on ATmega8535.
When you start program use CodeWizardAVR to generate basic program structure.
In our case we used ATmega8535 so we will choose that modem from Chip tab in wizard window.
For frequency chose correct frequency that is used (external xtal, calibrated internal RC oscillator..).