This game is too complex for a walkthrough...there is no perfect path to get to a girl.
Once you figure out what a girl likes and dislike ... It's pretty easy to get to them.
Having said that, there are a couple of girls that are 1) not that into you (Melissa, Astrid), 2) Already have boyfriends (Andrea, Tatiana) or 3) Are still pissed of with you from previous relationships (Irene, Kirsten)
Some girls are very open for relationships, while some are only looking for a fuck.
Keeping that in mind, you need to have a long play, medium play and short play in mind when going for the girls.
What girl are you looking for? I might be able to give some hints.
(If it's Melissa...you can not really get to her yet. There is a small path to her right at the end...)
Use common sense, Don't be to much of an asshole. Treat woman differently according to their personalities.