The witness, dubbed Victim No. 4 by prosecutors, said what began as "soap battles" in the shower escalated into inappropriate touching and oral sex.
The man, now 28, was the first of eight alleged victims expected to testify during the trial, which began Monday with opening statements.
Sandusky faces 52 criminal counts that he sexually abused 10 boys over 15 years, allegations he has denied. His arrest and the fallout led to departures of longtime football coach Joe Paterno and the university president.
The trial is expected to last several weeks.
Lead prosecutor Joseph McGettigan III opened Sandusky's highly anticipated trial Monday by telling jurors that the 68-year-old retired coach was a pedophile who took advantage of fatherless children or those with unstable home lives, plied them with gifts and sexually abused them for years.
Defense lawyer Joe Amendola countered that some of the alleged victims had hired civil lawyers and had a financial interest in pursuing the criminal case.
Sandusky sat still as the first witness explained that he began showering with the former assistant coach in 1997. The man said he had met Sandusky through The Second Mile, the children's charity the assistant coach had founded.
The witness spoke calmly and firmly when questioned by McGettigan. Wearing a white shirt, dark tie and dark pants, he looked straight ahead at McGettigan during questioning. He gestured at times when asked to describe interactions with Sandusky.
"He would put his hand on my leg, basically like I was his girlfriend ... it freaked me out extremely bad," the man said, extending his right arm out and pushing it back and forth.
"I pushed it away ... after a little while, it would come right back. That drove me nuts," he said.
Instances in the shower, the man testified, escalated to the point where either Sandusky maneuvered himself so his head would be near the boy's genitals, or vice versa. The man testified that there were "a few occasions" where Sandusky ejaculated in the Penn State locker room showers.
Pictures of Sandusky and the then-boy were shown at times on a video screen. The man was asked to identify photos handed to him by McGettigan, including those with Penn State football players, but rarely looked over when the pictures were displayed on a screen large enough for jurors to see.
The man said he stayed either at his mother's or grandmother's home at times. He never told his grandmother.
"No, no way. I was too scared to ... The other things were nice. I didn't want to lose that," he testified.
A self-described college football fan, the man said he enjoyed the access to Penn State football games and facilities. At one point, the man said, Sandusky let him wear the No. 11 uniform of LaVar Arrington. Prosecutors also showed a picture of the man, as a boy, with Arrington.
The man testified that Sandusky also took him to bowl trips including the Outback and Alamo bowls. He also gave him golf clubs, snowboards, drum sets and various Penn state memorabilia including a watch from the Orange Bowl, the man testified. He said he would wear gift jerseys to school.
The man said that, as he got older and after he got a girlfriend, he was "basically getting sick about what was happening to me."
He testified to one alleged interaction before a bowl game banquet in Texas, in a hotel bathroom before taking a shower, that Sandusky pushed down on him in a "downward motion."
The man said he resisted, when he testified that Sandusky responded, "You don't want to go back (home), do you?"
Asked by McGettigan to clarify, the man said "that he was trying to get me to have oral sex, and threatening me if not."
He said about 10 seconds later, Sandusky's wife, Dottie, called out from another room, and that an apparently surprised Sandusky left the bathroom.
Sandusky also sent thee man letters, the man testified. One shown briefly on a video screen in court was a handwritten on Penn State letterhead, signed "Jerry"
"I know that I have made my share of mistakes," the letter read. However I hope that I will be able to say that I cared. There has been love in my heart."
The man described some of the correspondence as "creepy love letters ... Others would be `Hey, do you want to come to a football game.' Those kinds of things.
Defense attorney Karl Rominger questioned the relevance of the letter, leading to a conference between the judge and attorneys at the bench. But questioning continued minutes later.
During his opening statement, McGettigan told jurors he would prove that the abuse included oral and anal sex involving boys Sandusky met through The Second Mile and that it took place "not over days, not over weeks, not even over months, but in some cases over years."
McGettigan called The Second Mile, which Sandusky established in 1977, the "perfect environment for the predatory pedophile" and his way to get close to his victims.
Amendola, Sandusky's lawyer, said the young men who would take the stand were accusers, not victims. He said jurors may find it odd that Sandusky showered with children, but that it was innocuous, and part of Sandusky's upbringing.
"In Jerry's culture, growing up in his generation, where he grew up, he's going to tell you it was routine for individuals to get showers together," Amendola said. "I suspect for those of you who might have been in athletics, it's routine."
Amendola said the defense will argue that Mike McQueary, the football team assistant who reported seeing Sandusky naked in a shower in 2001, was mistaken about what he saw.
"We don't think Michael McQueary lied," Amendola told jurors. "Are you surprised? We don't think that he lied. What we think is that he saw something and made assumptions."
Amendola also told jurors that at least six of the alleged victims have civil lawyers, including several in the courtroom gallery on Monday.
"These young men had a financial interest in this case and pursuing this case," Amendola said.