How to stop freezing up when approaching girls?

I often encounter women in public transport that glare at me. If I like the girl I try to look back and smile, sometimes they notice it sometimes not. When I got off the train today this happend while we were waiting for the train to stop and the doors to open. She looked directly at me and smiled, I smiled back looking into her eyes. Then she turned around and started playing with her hair, while glaring at me. After we got out she glared back a couple of times, probably waiting for me to make a move. I just froze and didn't knew what to do next, I wanted to talk to her but didn't knew what to say. Situations like this have happend to me multiple times and everytime I froze. I don't want to freeze anymore, I would like to talk to her and eventually ask her number or something. How do I stop this from happening and what can I say to her?
How to stop freezing up when approaching girls?
Post Opinion