According to the student pilot of the Cessna 172P, he was conducting a solo cross-country flight. After entering the airport traffic pattern on the downwind leg, he configured the airplane for landing and reduced the engine power to idle on the final leg of the approach. During the landing, the airplane "porpoised and landed hard" resulting in damage to the nose landing gear, propeller, and firewall. The pilot reported that there were no preimpact mechanical failures or malfunctions of the airplane.