Well unfortunately there are many situations.
I've had quite a bit of experience especially recently...
There are several types of approaches, the most common one at your age is when the girl shows interest in you, in a rather obvious way.
When I was 13 (> 7 years ago now
) that's how I had my first girlfriend, she was like "wow" at everything I did, she wrote me letters, etc. you get the idea.
There are also girls who you've never met or noticed, and suddenly ask you out. Hate them
how are you supposed to date someone you've never talked to before? Feels extremely uncomfortable when they ask.
Lately (actually a few months ago) I've been going out with a girl who I didn't know before, and I had no idea she was into me as much as I was into her. I hadn't noticed any interest on her side, but I was very attracted so I just asked her out and it turns out she was in love with me since the first time we met
So.. you see, girls can be quiet but have feelings nonetheless! Maybe that's a situation you wouldn't face at your age, and anyway I don't know how it works in Denmark!