Eh, just start keeping a notepad or spreadsheet with what works for you. You can dig into theory and calculations as you go, but it's super important to start now and stay with it. Without your own notes it's too easy to fall prey to flavour-of-the-day marketing.
My spreadsheet is not terribly complicated; I just keep track of the lower triangle defined by axles and bottom bracket and the upper triangle defined by touch points. No bike theory required. You do need Heron's formula for BB drop, but that's just 3000 year old triangle geometry
Yup, or my approach, which is buy what seems right, then, occasionally, I have to adjust the seatpost setback and/or stem length.
And in today's market, I believe my approach to have more real world applicability!