How, when and if you need to deal with the local authority (LA) with regards to home-education depends on your situation.
If your child has never been to school, there is NO obligation to inform the local authority of your intention to home-educate. You simply do not apply for a school place.
If you have applied for a reception place but have changed your mind, you need to withdraw your application or your acceptance (if you have already accepted a place).
If your child is already in school (not a special school, as below), then you will need to deregister your child from that school.
If your child is in a special school (this includes an independent special school arranged/paid for by the LA), then the parent(s) need to seek consent from the LA to deregister.
If your child is in an independent special school that is arranged and paid for by the parents, consent from the LA is not required.
The government guidelines for local authorities with regards to elective home education can be downloaded here.