Negotiations began yesterday for a new contract covering 26,000 janitors, doormen, elevator operators and repair workers at more than 1,000 office buildings in Manhattan, Brooklyn and Queens. Mike Fishman, the president of the city’s largest union of building service workers, Local 32BJ of the Service Employees International Union, said its goal was to bring workers’ wages in line with the cost of living at a time when the city’s office rents have been rising. Local 32BJ is negotiating with the Realty Advisory Board, which represents office building owners and management companies. The workers’ current contract expires on Jan. 1.