Today's action at the Asics Junior National and Advocare Cadet National Championships saw America's best Junior Greco-Roman wrestlers take the mat for the first time while the Cadet Greco-Romans took another step toward the finals at the FargoDome in Fargo, N.D.    The Junior Nationals opened with 792 Greco-Roman wrestlers going head to head with hopes of still being alive for the finals Tuesday night when two wrestlers from each weight class will battle for the gold. Several 2000 champions started strong on their quest for a repeat.      After winning his first match by forfeit, defending Junior champion Harry Lester of Ohio took the mat by storm by manhandling local favorite Marc Mellmer of North Dakota with an 11-0 technical fall in only 27 seconds in the 132-pound weight class. He won his third match of the day in similar fashion with a 10-0 technical fall.     Defending champion Cam Jones of Utah didn't waste any time either with three wins by technical fall by scores of 13-0, 11-0 and 13-3 at 132 pounds.    Matt Nagel, who moved up a weight class from his 2000 championship weight of 143 pounds, had no trouble at 154 pounds with three wins by technical fall, allowing no points.      It was the same story with the second day of Cadet competition, as the returning champions continued dominating their opponents.     Grant Miller of Illinois has yet to be scored upon, winning two matches by an 11-0 technical fall and a pin in 42 seconds to improve his record to 5-0 at 209 pounds.     Returning heavyweight champion, Patrick Barrentine of Ohio also lived up to his champion status, allowing one point in four victories that included a technical fall and a pin in 27 seconds.    Returning runner-up Steve Anceravage of Pennsylvania didn't fare as well, though, getting pinned by Eric Luedke of Kansas in 2:19 for his second loss and a disappointing elimination from the 132-pound weight class. Sean Santana, who beat Anceravage by a referee's decision in the first day of competition, won four matches today with two by technical fall, leaving him with a record of 7-0.    Competition resumes Monday with round two of Junior Greco-Roman competition and the final preliminary round and medal matches at the Cadet Division.