1. if i do not have a job , do i get the application for the population registry form when i enter spain?You can then travel to Spain, your wife needs to go with you. If you go there first, your wife can still apply for this visa to join you but must show that you are there, either you've been there for less than three months or you've started to exercise treaty rights, in this case the job there.
Once there you both need to enter the population registry, which is a formality, and you might be given a registration certificate. Here practices vary between member states and I don't know the Spanish rules.
2. if i start a job one day after i enter spain, do i still have to enter the population registry immediately?