Yankees superstar Alex Rodriguez, in his first press conference since he admitted that he used steroids earlier in his stellar baseball career, said Tuesday that he "screwed up big time" by taking drugs.

The decision was a "stupid mistake and a lesson learned for a guy with a lot of baseball left to play," said Rodriguez, who last week admitted to using steroids during the 2001-03 seasons.

A-Rod, the Yankees' third baseman, said he injected the substance twice a month for six months — or at least 36 times — during the three seasons, when he played shortstop for the Texas Rangers.

He said he received the substance from his cousin, whom he declined to identify. He said the banned drug was obtained over-the-counter in the Dominican Republic.

"I stopped taking it in 2003 and I haven't taken it since," said Rodriguez, claiming that he was unaware what he was injecting into his body.

"I didn't think they were steroids," he said. "It was really amateur hour. All these years, I never thought I did anything wrong."

But he admitted: "I knew we weren't taking Tic Tacs."

A-Rod, a three-time American League Most Valuable Player, said spring training represents a "new start" for him.

"Like everyone else, I made a lot of mistakes in my life," he said. "One thing I know for sure is that baseball is a lot bigger than Alex Rodriguez."

Rodriguez — baseball's highest-paid player — stopped short of saying he shamed the game of baseball, but he acknowledged the long road he faces to redemption 10 days after Sports Illustrated reported that he tested positive for steroids in 2003.

"I'm here to take my medicine," he said.

Rodriguez, who promised teammates the "best season" of their lives, sidestepped questions as to how the three seasons in question should be viewed by baseball purists. He is widely expected to pass Barry Bonds' record of 762 home runs and become baseball's home run king before he retires.

"To what degree it helped, I'm not sure," he said. "But after today, I hope to put this behind me and focus on baseball."

FOXNews.com's Joshua Rhett Miller contributed to this report.

Click here to read Rodriguez's official MLB biography.