I just started today. Do I actually need to raise affection points for every girls or focus on the girls I like?
There are a couple of times where raising the stats of girls you like gives you a little extra, but the game is designed to have a way of you seeing every event / scene in the game. General consensus is viewing every event (green path) is the intended way of experiencing the game.
Sometimes you might even want to consider not viewing an event for a while to grind up stats. When you've viewed every character event available the game will proceed with main events that will progress the chapter. If you need to do some grinding that's how you give yourself the space.
Grinding tip: If you have spare days probably in chapter 2 that you're just trying to mill through to the night or something, grind for
Osako, Chinami and Tsubasa affection in that order, at least to 30. You can reach points where you need 20 extra affection for them before you can see their next event, and you can only earn 2 per week due to the probably afternoon timeslot needed for them.
Highly recommend the guide mod, AutoVN is there if affection grinding would put you off of the game otherwise, but you will forget that side content exists.
You should know about URM and how it can be used to skip / fix atrocious problems in the game, but never use it to make dialogue choices / view alternate dialogue paths without rolling back before the split and letting it play out naturally. You can break your saves and make diagnosis impossible since checks for missed events are randomly placed inside unrelated events.
You should also block "bonus" from being monitored as a variable unless you want to read twice as much text and click several hundred times, it governs the old SFW mode LiL had for a while. You'll miss out on some random lore but you could always ask about it here.