Make json string from Spark Dataset

I'm very new to Spark/Scala. Given a spark dataset, what's the proper way to construct json string from it. For example:

case class MyDataset(a: String, b: String, c: integer)

val ds = sparkSession.sql("select a, b , c from tbl").as[MyDataset]

|   a |  b |  c |
|0060Z|  NY|  16|
|0060Z|  LA|  23|    
|6060R|  GA|  12|    

I want to use ds.foreach and construct something like this: