Hi, Anwar
Welcome to AEM Community.
Here are the good resources to start with.
As a starter, I hope you have minimal knowledge on html, css, JavaScript (+Frameworks), and Java. Even though you want be a back-end developer, I suggest you to lean basics in UI technologies.
The official documentation from adobe is - https://docs.adobe.com/content/docs/en/aem/6-2.html
The how-to articles (practical implementations) - https://helpx.adobe.com/experience-manager/topics/how-to.html
The how-to videos from community - https://www.youtube.com/channel/UCDtISU9W-zzo1PIU3pGHUbA
These are pretty good to start. Happy learning!
Thanks,
Vamsi