Test run and initialization

Hi,

I am trying to figure out, how to get started.  I want to get a sip trunk connection and number to test out on PBX system.  I've noticed a lot of parameters, but I am simply interested in getting 4 simultaneous channels at best and use my PBX system. 

What should I do first?

Configure the PBX and then set up the trunk?  or set up the trunk and number and then get the PBX system configured? 

Any thoughts?