If you live outside Hertfordshire, you must apply to your own council. That's even if you're applying for Hertfordshire schools.
Here are the contact details for the council bordering Hertfordshire.
Barnet London Borough
Bedford Borough Council
Buckinghamshire County Council
Cambridgeshire County Council
Central Bedfordshire Council
Enfield Council
Essex County Council
Harrow Council
Hillingdon Council
Luton Borough Council