First you need to have an apple account that you can open in:
Once logged in, you can activate your developer account at https://developer.apple.com :
After reading and accepting the conditions, you have to join the apple development program.
You can enroll as an individual or as a company.
If you enroll as an organization you will need the DUNS number and be legally incorporated.
If you roll as an individual these documents are not necessary.
And you can pay the amount of your subscription for $ 99 / year.