A 12-YEAR-OLD girl became a prostitute to help her boyfriend pay a gambling debt, the District Court heard yesterday. The court heard that a female friend persuaded the girl to start taking on ''customers'', telling her she could make up to $18,000 for her first time. The friend, Tong Ka-man, 20, is accused along with the 12-year-old's boyfriend, Wong Ho-tai, 17, of conspiring to procure a woman under 21 and living on the earnings of prostitution. In an opening address, prosecutor Simon Lam said that on May 14, 1992, the girl met Tong and Wong in Yau Ma Tei and saw that Wong had been beaten up. Tong explained that Wong owed money in Macau, and suggested the girl pitch in and help pay his debts. ''She asked me to receive customers - to have sexual relations with other people,'' the girl testified. ''I paused to consider the matter for a while and I agreed.'' About a week later, Wong and another man escorted the girl to a Yau Ma Tei restaurant, where she was introduced to 47-year-old Leung Kit-chiu. Leung brought the girl back to his home in Yau Yat Chuen. The girl testified that the pair had sexual intercourse, and then took a bath together before retiring to the bedroom. They had sex a second time before going to sleep. Leung has been charged with having unlawful sex with a girl under 13. The crown alleged that Leung paid a man who took the girl to see clients $18,000 in two instalments, and that the money was later passed on to Tong. On May 23, Tong arranged for a man to take the girl to a Shamshuipo flat to ''receive customers''. Several clients later, the man took the girl back to a bungalow on Lantau Island, where Tong was waiting for the day's take, the prosecutor said. The next day, the girl was again escorted to the Shamshuipo flat, where she received five customers while the man waited. With money in his pocket, the man again steered the girl to the Lantau ferry, but the pair missed the last boat and were caught by police. The trial, under Judge Jackson, continues.