I'm trying to figure out the whole "global chat" thing...what is the command to "tell" via the global chat??

/tell hero_name@user_name, message

If someone /tells you, you can reply by clicking backspace.

Further instructions here? I tried sending a reply last night to someone who sent me a tell, and when I hit the backspace key, I got nothing.

When you say "Click" is there a back-space button on the screen?

Your reply key depends on your keybinds and how you setup the defaults. For some people it's R, for others its Backspace.
You can change it in the keybind menu under Options.

You'll find it easier to also just send a /tell to just @account, instead of CharacterName@account. It will auto-send the tell to whatever character they're logged in as. Mail works the same way.

or you can just use...
/t @username <message here>

It works just the same as /tell hero_name@user_name <message here> but with less typing, plus it will send to them no matter what character they are on.