An interview conducted with the 28-year-old former Sunderland winger after his arrest was read out at his trial at Bradford Crown Court.
In it, Johnson admitted exchanging messages with the girl on social media before meeting her to sign football shirts.
He then admitted kissing her at a second meeting on 30 January last year, but denied it was passionate and repeatedly denied more serious sexual conduct.
Johnson told the detectives: "I knew it was wrong. I have a girlfriend for one. Lots of reasons. I've got a family, a career."
He told the police: "I said to myself 'it was poor, like, I should have known better'."
Johnson said: "The worst day of my life. I was trying to do a good deed which has turned out to be ... you know? I feel terrible, absolutely terrible."
During the interview, the player denied undoing the teenager's jeans or touching her intimately. He said: "I did not touch her down her pants or anything like that."
Johnson agreed he met the girl after they had talked on WhatsApp about a 'thank you kiss' for him signing the shirts.
"She came into the car and she said about a thank you kiss," he said. "It was just like a normal kiss. Just how you would kiss someone."
Asked if it was a 'peck', he said: "It wasn't a peck, it was a kiss." Johnson told the police it was not passionate but did involve tongues.
The interviewing officer, Dc Kimberley Walton, asked the footballer why, when he was arrested, he told his girlfriend, Stacey Flounders, the teenager told him she was 16.
Johnson said he was just trying to keep the truth from Miss Flounders and he was not trying to deceive the police.
He said: "It was more between me and her. I didn't want to go back home and she'd taken my daughter and didn't give me a chance to explain myself.
"I sort of just panicked. I wasn't going to keep it from you (the police)."
Earlier, Johnson admitted he had created a new Snapchat account to talk to the teenager so Miss Flounders could not see it.
He said they had had problems in the past with him 'texting girls and stuff'. He said: "It was more to hide it from my girlfriend, if you know what I mean."
The jury of eight women and four men has heard Johnson has admitted kissing a 15-year-old girl in his Range Rover on 30 January.
He has also admitted grooming the girl but denies more serious sexual conduct took place between the pair in his car.
Dr Paul Hunton, a forensic computers expert, told the jury a Google search for 'legal age of consent' was made on Johnson's phone four days after he met the girl on 30 January.
He described how he examined an iPhone 6 from Johnson's house and verified a series of web pages were accessed on 3 February last year.
Dr Hunton said the Google search for 'legal age of consent' was followed by another web address which appeared to show the user had clicked on a result which took him to the www.mirror.co.uk website.
He said the search result clicked on appeared to be 'What is the age of consent around the world? From Angola at 12 to Bahrain at 21'.
The expert said the next webpage accessed on the phone appeared to be another search result, this time on the BBC website.
He said the link was titled 'BBC Radio 1. BBC Advice. Age of Consent'.
Johnson sat in the dock wearing a blue suit, white shirt and grey tie, listening to the evidence.
The player has pleaded guilty to one count of grooming the teenager and another of sexual activity with a child.
He denies two further counts of sexual activity with a child.
The trial was adjourned until Monday.