The duty cycle is set by writing to the OCR1AH/L register pair (The equivalent of CCPRxL and DCB1 : DCB0 on a PIC)...NOT the TCNT register pair. However, in AVR-GCC, you can just write to OCR1A and the compiler takes care of the register pairing.
TCNT1H/L is the timer counter itself. These registers need not be written to.
OCR1AH/L holds the duty cycle value itself. In 10-bit PWM mode, you can write a value from 0x0000 - 0x03FF. A value of 0x1FF or 0x200 should get you close to a 50% duty cycle -
Code:
OCR1A = 0x1FF; // 50% duty cycle
As for setting the period, this is done with the prescaler setting/clock source select bits.