With no context whatsoever we can only advice you to say stuff like "hello"
But in my opinion usually the best conversation openers are those were you mention something you've observed about them. It shows more of an interest and is less likely to cause an awkward pause with a loss of what to say after the hello.
It's stuff like , "oh that phone case , I love that show. What else do you watch?" that are more comfortable and end up starting a long convo. One of those that are so engaging that you forget to ask their name until the end.
But hello is good too if you can easily carry it further