This is one way:
const byte LED = 10; // Timer 1 "B" output: OC1B
// 16 MHz clock divided by 18 KHz frequency desired
const long timer1_OCR1A_Setting = 16000000L / 18000L;
void setup()
{
pinMode (LED, OUTPUT);
// set up Timer 1 - gives us 18 KHz
// Fast PWM top at OCR1A
TCCR1A = _BV (WGM10) | _BV (WGM11) | _BV (COM1B1); // fast PWM, clear OC1B on compare
TCCR1B = _BV (WGM12) | _BV (WGM13) | _BV (CS10); // fast PWM, no prescaler
OCR1A = timer1_OCR1A_Setting - 1; // zero relative
OCR1B = (timer1_OCR1A_Setting / 2) - 1; // 50% duty cycle
} // end of setup
void loop() {}
Output is 18 KHz:
Frequency analysis (there are harmonics, which you expect with a square wave):
