How do you talk in general chat?
Solution 1:
It's /c (for channel)
/c Hello there.
This, of course, requires you to actually be in the General channel. If you joined another channel the same command would let you speak there, and if you're not in a channel the command wouldn't work.
You can also press the Quick Chat button next to the chat box, or just hit the [tab] key with chat open, to choose where your messages will be sent.