How to set trunk outgoing host?

I am beginning asterisk user bk.

I install asteriskNOW and I success internal call.
but I can’t call other proxy server and endpoint.

I think my trunk setting has problem. please
advice for host ip and domain

trunk name : <-- asterisk installed server domain.
is it right?
host= <-- asterisk installed servier ip.
username=8000 <-- I made extension.

some people say host is software switch server… is it right?
my coworker say host is asterisk installed server ip.

and other person say host is other proxy server ip. you want to connect.

What is right? this is too difficult.