I started out by using trixbox. It is very helpful and a easy way to get to understand asterisk. You may also want to find the book " Asterisk - The future of telephony" YOu can pick it up at any bookstore or you can download the pdf online. Also check the wiki www.voip-info.org tons of info there. Stay reading and you will catch on in no time.
Simply put, if you use Trixbox you have no job security. Any idiot (not calling you one) can use a web GUI but it takes lots of learning to use Asterisk properly.
As bjweeks said above trixbox would not be the way to go. Although you can learn a little bit, you will become dependant on the GUI and not really learn asterisk “the right way”. Anyone can use a GUI. It takes a lot more to know the actual code. When I first started with asterisk I too looked for a GUI and on the lists they told me that if I wanted to do it the right way then I should learn the “real way to do it”. I can say that that was the best advice that I recieved.
not forgetting that before too long, you’ll see the limitations of the dialplan the GUI forces on you. i doubt 2 people set up a PBX in the same way given enough options and flexibility, so why limit yourself to what the GUI devs have come up with.
although my biggest reason for ditching AMP was that using the version i was running with Asterisk 1.2.x produced a heap of errors. coupled to the fact that it needed lots of mods to work in the UK and the features i wanted/envisaged meant i was going to write most of the dialplan anyway, it was consigned to an archive directory.
and my /etc/asterisk directory is now a lot smaller, makes easier reading, and processes calls more quickly.
if you want to free yourself of the limitations, use a GUI as a quick way to get familiar with the technology, write your own addons/web-interfaces/agi scripts, get stuck into the config files and you’ll wonder why not everyone is as enlightened as you