What Type of SE Is Most Appropriate for Beginner?

Hiya again.

I read few SE articles. It is amazing practice!

Situation: I have email address; IP address changes by every mail I receive; password is unknown; target is one email address; can SE by sending email to second email address that IS currently used to get access to older email address.

Question: What Social Engineering technique/article should I read/get actuated with to successfully acquire password for "older" email address?

Many thanks!