Looking to develop a career in IT

I applied for an IT Support Specialist for USPS and unfortunately didn't get it. I was able to reach the hiring manager exchange some emails with him. He mentioned that having knowledge in Active Directory and Office 365 would help me.

For background, I have always messed around with computers and I self taught myself HTML, CSS, and some JavaScript as well as Figma for UI/UX. I don't have any work experience in the tech field which is why I think IT is a good way to get myself in this career path.

My question is where would I go to gain knowledge on backend support for programs? Most of my knowledge has been front end so I'm a little lost on where to go/start.