Your problem is not "what to say". Girls don't care what you say. It's about the energy you bring to the conversation. And that energy is dictated by a variety of things. To name a few:
1. How invested you are in this particular girl? <---If this is cold approach, hopefully not much.
2. How outcome independent you are.
3. How you're feeling that day.
4. How much social momentum you have.
5. How needy you are around women in general.
6. How much experience you have with women.
7. Your general mental health
...and the list goes on.
So here's my question to you...when you go out talking to these girls
how do you feel? Because if you feel anxious, awkward, uncomfortable or any other negative emotion, she'll feel it too (if you don't believe me, look up "mirror neurons" on google). And if you're giving a girl negative emotions, it doesn't matter what your conversational topic is, she won't be into it. Conversely, if you're feeling positive, you can be having a pretty dull conversation, and she'll probably be pretty receptive because you're giving her good feelings.
P.S.
You already know what to talk about. You've been having conversations for the past 18+ years (however old you are). So I highly doubt figuring out "what to say" is a problem for you. Your problem is that you judge and critique every possible conversational topic that comes to mind as "not good enough" (whether this is a conscious process for you or not, I don't know). But here's a little rule I like to use to help with that:
Say whatever comes to mind. I don't care if you think its stupid. Just say it. You'll be surprised how often girls will be amused or receptive to something you think is retarded. Plus, it's better than saying nothing at all or awkwardly trying to think of conversation topics.
Also,
I did some mall game and approached six girls
This isn't nearly enough. Six approaches is about the time when you'll start finally building up social momentum and getting out of your head. Everything up till then is what I would normally consider "warming up". Had you continued your outing after this, I'm willing to bet your interactions from then on out would have been better.