Ninth-graders search out the news, First the Good News, by Judie Angell. Scarsdale, N.Y.: Bradbury Press. 147 pp. $9.95. Ages 12 and up.

''First the Good News,'' is a satisfying ethnic Nancy Drew-type thriller set in contemporary New York. A mystery it's not. There's no crime to solve - only a dilemma: How can a group of ninth-grade girls wangle an interview with the hottest TV idol and win the interschool journalism award?

These bright and independent girls have formed a group called the ''Adam's Ribbers,'' to work for the ''common good of mankind.'' While all too often it seems their time is taken up with arguing about the color of their club sweater, they manage to squeeze in such projects as working in a soup kitchen, singing Christmas carols at a home for the elderly, and writing columns on world affairs.

So where does stalking celebrity Hap Rhysbeck fit in this plan? Josephine says, ''There's something he gives people. . . . It's more than just being funny. He speaks to kids. The way our parents and our teachers don't.''

The teen's speech rings true - a difficult feat in the face of their constantly changing speech habits. The parent's dialogue, however, goes whole hog into ethnic stereotypes. It's not a major flaw; in fact, the ethnicity of the story is part of its charm.

How these five friends manage to cope with protective doormen and prying students, keeping their morale up (and their weight down), plus trying to figure out the identity of the suitor, makes for fast and entertaining reading.