Oh, I think I misunderstood your question :)
Well, it depends on the context. Some examples:
- Hello, I'm your new neighbour and I wanted to introduce myself.
- Hi, I'm lost, could you please help me find my way?
- Good morning, your dog is very cute!
Basically, you want to find a reason to start a conversation with a person you don't know. And depending on that reason, the first sentence can be completely different!
In the top menu, click on Community, Language Partners, enter your search criteria and find someone you like. Then click on their picture, and on the Message button.
Well, it might be weird, but I usually ask the person if she wants a cake (or candies , it depends on what I have in my bag)! I eat a lot and always have something to share :D It's like "Do you want a candy? Ho and what's your name? " haha.
"Hello, stranger" by Closer movie. :P
I'll assume you're referring to a conversation on this site. So I'd start by discussing the reasons you & the other person have for learning a new language... how long have you each been at it, what are your goals, what learning path have you followed, etc. Hopefully you'll find some common interests beyond language but I think too many people hope or assume that they'll find friends through this process and are disillusioned when that doesn't happen. So assume that it does not and the only connection you develop with this person is that you both want to learn a language. That recognition alone could put an end to the so-called "dating" or "love interest" aspect that people think they will find here.
I would plan the next conversation to be based on a shared discussion about an article that you each find on the net. You would be tasked with finding an article in your native language & they would do the same. Email/message the link to the other person & read both articles before the next time you meet. Now you've got something specific to talk about & in time you may find enough common interests that it flows on its own.