Think likethe Microcontroller

// Blink the LED on D13 (PB5).
// Hold it on while the button on D8 (PB0) is pressed.
// ATmega328P at 16 MHz, bare-metal C: no Arduino library.
#define F_CPU 16000000UL

#include <avr/io.h>
#include <util/delay.h>

int main(void)
{
    DDRB  |= (1 << DDB5);      // PB5 (LED) as output
    DDRB  &= ~(1 << DDB0);     // PB0 (button) as input
    PORTB |= (1 << PORTB0);    // turn on PB0's internal pull-up

    while (1) {
        if (!(PINB & (1 << PINB0))) {   // pressed: the button pulls PB0 to 0
            PORTB |= (1 << PORTB5);     // hold the LED on
        } else {
            PORTB ^= (1 << PORTB5);     // not pressed: toggle the LED
        }
        _delay_ms(500);
    }
}