I want to design my first PCB from schematic to finish, what do I need to learn first?
Start simple, do some ready made projects first and add more of your own effort into the design chain each time.

First use ready board layout (learn PCB fabrication or ordering)

Then ready schematic (learn PCB layout)

Then ready idea (learn schematic capture)

Then all your own (learn about data-sheets).
Learn a new tool each time.