df.iloc[:0] # Give me all the rows at column position 0

df = sc.parallelize([(1, "foo", 2.0)]).toDF()
df.select(*df.columns[:1])  # I assume [:1] is what you really want
## DataFrame[_1: bigint]

df.select(df.columns[1:3])
## DataFrame[_2: string, _3: double]

val df = sc.parallelize(Seq((1, "foo", 2.0))).toDF()
df.select(df.columns.slice(0, 1).map(col(_)): _*)

scala>: df.columns
Array[String] = Array(age, name)

scala>: df.select(df.columns(0)).show()