IBM and Sears, Roebuck will launch their long-awaited videotex service next year in San Francisco and several other cities, a spokesman said. The pair have invested an estimated $250 million in Trintex, a joint venture that would deliver information, shopping and brokerage services to consumers by personal computer. Other markets will be chosen for the service, called Prodigy, in 1988, with nationwide availability to follow in the next few years, the spokesman said. Analysts say Prodigy will need several million subscribers to break even. Trintex will charge a flat monthly fee for Prodigy, similar to basic cable TV charges.