General tips: Assume Lizzie is listening in all scenes and say nice things. It's okay to disagree with her, she's self-aware. Don't choose too many romantic options in a row or she'll figure out you're manipulating her.
Not gonna do a whole guide, but it helps to understand the structure of the game. The game doesn't choose your route based on flags, but variables. The average decisions you make change these variables, while the big branches are at certain points based on your variables so far.
For example, when Lizzie invites you to her house, different things can happen. If you have sex with her, you go down the "girlfriend" branch. If not, you do the "kidnapping" branch. If your variables suck, you might be forced onto the kidnapping branch without a choice.
Both of those branches split into two as well, leading to four different ending scenarios. If you do well on the girlfriend route, you can stop Lizzie from killing your lab partner. If you choose the right options in the basement, you can get the chance to escape but stay instead.
It gets trickier from there, because there are lots of other details. For example, if you talk to teachers and act suspicious, then you'll get in trouble on some of the branches (otherwise you can safely avoid attention).
The true ending is on the kidnapping path, staying with Lizzie, then not running away from school. But if your stats are bad, John dies instead of helping Lizzie go full yandere.