Answer by taking a quote from the following article:

Jens Ingemar Johansson (Swedish: ['ING:emar 2ju:hans,son]; 22 September 1932 - 30 January 2009) was a Swedish professional boxer who competed from 1952 to 1963. He held the world heavyweight title from 1959 to 1960, and was the fifth heavyweight champion born outside the United States. Johansson won the title by defeating Floyd Patterson via third-round stoppage, after flooring him seven times in that round. For this achievement, Johansson was awarded the Hickok Belt as top professional athlete of the year--the only non-American to do so in the belt's entire 27-year existence--and was named the Associated Press Male Athlete of the Year and Sports Illustrated Sportsman of the Year.

Johansson's introduction to the top rank of the sport was inauspicious. At age nineteen he was disqualified for passivity at the Helsinki 1952 Summer Olympics in the heavyweight competition in a fight against eventual Olympic gold medalist Ed Sanders. Johansson maintained he was not evading Sanders (who also got a warning for passivity), but rather was trying to tire his opponent. Johansson said he had been limited to a 10-day training camp, had only trained with newcomers, and had been told by his coach to let Sanders be the aggressor. Nevertheless, his silver medal was withheld for poor performance and only presented to him in 1982.  Johansson had earned his spot in the Olympics by winning the Swedish National Championship earlier the same year, 1952, after he knocked out his opponent in the first round of the final.  After the Olympics Johansson went into seclusion for six months and considered quitting boxing. However, he returned to the ring and turned professional under the guidance of the Swedish publisher and boxing promoter Edwin Ahlquist, subsequently winning his first 21 professional fights. He won the Scandinavian pro title by knocking down and outscoring the Dane Erik Jensen (breaking his right hand in the process). A broken hand and a one-year military service kept him out of the ring until late 1954. In August 1955, in his twelfth professional fight, Johansson knocked out former European Heavyweight Champion Hein ten Hoff in the first round. He took the Scandinavian heavyweight title in 1953 and, on 30 September 1956, he won the European Heavyweight Championship by scoring a 13th-round KO over Italy's Franco Cavicchi in Milan for the European title.  Johansson successfully defended his European Crown against ranked heavyweights Henry Cooper (fifth-round KO on 19 May 1957) and Joe Erskine, with a TKO in 13 on 21 February 1958.

How did he get started in boxing?
At age nineteen he was disqualified for passivity at the Helsinki 1952 Summer Olympics in the heavyweight competition