JONI MITCHELL, a current happening on the national scene, not only sings folk songs, she writes them. In fact, she's rated as one of the top contemporary folk composers in the business. Her tender ballad, "Both Sides Now", was successfully recorded by such artists as Frank Sinatra and Judy Collins. She'll sing it herself at 9:30 tonight when she appears as a guest on the new ABC "Johnny Cash Show" on Channel 7.

Joni began to catch on as a singer early this year when she drew raves at the Miami Pop Festival. A native of Saskatchewan, she didn't really become interested in music until five years ago when she was an art student at Calgary and casually started toying with the ukulele. Then she drifted into folk music and married Detroit folk singer Chuck Mitchell. Before their divorce the couple appeared together in Detroit area night clubs. Joni bears a strong resemblance to Mia Farrow and wears her blond hair long and straight.