I’m in the US, so I don’t know the customs where you are, but if I were a young lady I would feel very strange if you approached me and started a long conversation if you had never acknowledged me before. But I would feel a lot more comfortable if you were someone I had seen there often, enough that we might nod or smile or even say “hello.”
After a few “hello” encounters, that might be the time to say something like, “By the way, my name is Andre. What’s yours?” and let it go from there.
And even if you do have a conversation and it doesn’t really go anywhere, I would suggest that you still continue to say “hi.” Even if you don’t get a girlfriend, you still have a friendly face at church, and she might know someone to introduce you to.