After Reset, Outputs going HIGH

Using Arduino LEDs and Multiplexing
#1

Hey,
I have a Problem with my Arduino Uno.
After I Reset or Start the Arduino, he Put the Output after 1sek on, without clicking the Button.

void Loop()
{
if (buttonState == HIGH) {
digitalWrite(ledPin, HIGH);
}
}

When the button is pushed, the led goes on after reset and 1sek, and when the button is not pushed, the led goas although on after 1sek.
The Arduino Switches without the button.

I dont know what is wrong.

#2

Post all your code. Use code tags. Post a schematic showing how you connected everything. Hand drawn is ok.

My guess would be that your input pin is floating and at the moment it is reading high all the time.

#3