It depends. The 1900s (1901-1999) was a century not a decade. So you just say "the nineteen-hundreds" and refer to "the early 1900s," "between 1900 and 1910," the first decade of, "the forties" or "fifties," and so forth.
You can simply refer to it as "the 1900s". You could say "the first decade of the 20th century", but it might appear somewhat long-winded. Generally, the former is accepted.
