The first states in Nigeria were created on May 27, 1967, by General Yakubu Gowon (then Head of State). Before that,

The first states in Nigeria were created on May 27, 1967, by General Yakubu Gowon (then Head of State). Before that, Nigeria was divided into regions (three at independence in 1960, then four after the Mid-Western Region was carved out in 1963).
There was no single “first state” created one at a time—all 12 states were established simultaneously on that date when the regional system was abolished and replaced with a state structure. These 12 were:
North-Western State
North-Eastern State
Kano State
North-Central State
Benue-Plateau State
Kwara State
Western State
Lagos State
Mid-Western State (later Bendel)
East-Central State
South-Eastern State
Rivers State
Some sources note slight date variations (e.g., May 5 or Decree announcement), but the widely accepted and official date for the creation of states in Nigeria is May 27, 1967.
If you’re referring to the oldest surviving states from that batch (after later subdivisions), they include Lagos State, Kano State, Kwara State, Rivers State, Kaduna State (from North-Central), and Cross River State (from South-Eastern).
(Note: The Mid-Western Region created in 1963 is sometimes called the “first,” but it was technically a region, not a state in the modern sense—state creation proper began in 1967.)