just bookmark this, it is free: https://www.365chess.com/opening.php
Where do you find main lines of openings to memorize

Schiller's books are mostly copy and paste--some flagrant plagiarism, some ChessBase stats.

I use a free chess database, and first learn the line that involves the most popular move at each ply. If a move has alternatives that are very close to equal in frequency then I learn all those, too. I usually make the cut-off frequency 50%, meaning I ignore moves seen less than 50% of the time unless there is a reason to include them. One such reason is that a program or human player I play uses that move frequently, or if it's an opening trap I should know, or (of course) if it's a little-known move that I've adopted into my repertoire.
"It is important for club players to build up a suitable opening repertoire. ..." - GM Artur Yusupov (2010)
"... For new players, I cannot recommend books that use [an encyclopedic] type of presentation [of opening theory], because the explanatory prose that elaborates typical plans and ideas is usually absent, thus leaving the student without any clear idea why certain moves are played or even preferred over other apparently equivalent moves. ..." - FM Carsten Hansen (2010)
"... [Fundamental Chess Openings] is not particularly suited for players who are just starting out. I would imagine players rated at least 1400-1500 would get the most benefit from this volume. ..." - FM Carsten Hansen (2010)
https://web.archive.org/web/20140626173432/http://www.chesscafe.com/text/hansen128.pdf
http://www.gambitbooks.com/pdfs/FCO_Fundamental_Chess_Openings.pdf
I would recommend Fundamental Chess Openings or FCO as it gives some lines and explains them too, which you probably need