Few thoughts - Start with basic web development languages html, css, js, jquery and Java. More items to look into - Java content repositories, OSGi, SLING and AJAX
Also see - https://experienceleague.adobe.com/docs/experience-manager-64/developing/home.html?lang=en#devtools
Try hands on component development and so on.
And of course follow this 🙂 https://experienceleaguecommunities.adobe.com/t5/adobe-experience-manager/ct-p/adobe-experience-mana...
There are probably some posts in the forum regarding getting started with AEM development.
AEM is Java Based Web Application
AEM Developer should have knowledge on the following Technology Stack
4) Java Script
AEM Developer should understand the AEM Technology Stack
3) JCR API
Please go through All getting started Adobe documentation and WKND tutorial