Solid answer by @Bittrance already.

My main highlights for a new to all this 'VoIP guy for a small office' would be:

Server setup:


Consider buying an appliance with a nice GUI. Several vendors make 'Asterix bundled with hardware' packages, like the "Asterisk Appliance 50" or "Trixbox".
If not an appliance, then at least consider a nice virtual machine / server software image (OS + Asterix + Addons + GUI) like AsteriskNOW, Trixbox, etc.


Upstream connection:


You can use an IP based telephony service provider, i.e. send SIP VoIP data over the T1 to your telephony service provider. This is where much of the business innovation, low telephony rates etc can be had. In a good design, you want QoS on the upstream (T1) line, to guarantee good voice quality in all conditions.
You can use a traditional telephony service provider, and use ISDN/2, ISDN/30, or multiple analog lines to connect to your upstream service provider. This is very reliable, time-proven, and doesn't have any QoS needs as you are not mixing data and voice on the same physical circuit.


QoS:


"Quality of Service (QoS) is the biggest design issue. QoS, at very high level, refers to the ability to prioritize data flows though shared pipes, i.e. make sure voice traffic gets priority over computer data.
How to best implement QoS in a given network is very dependent on what equipment is being used.
In a few cases, the simplest/cheapest QoS solution is to just use 2 physical circuits -- one for data, and another for voice. For example, some small offices use a new high-speed FiOS package for Internet connectivity/data, and an old T1 or something for voice uplink. The most cost-effective option will depend on what is being offered in your area, and the QoS capabilities of your existing firewall/router.
You should go for QoS on your internal network too. That's not so hard; the common solution is setting up dedicated VLANs for voice and data, and using the QoS implementations in your (managed Cisco) switch. Some really cheap installations use 2 switches, one dump ~30 USD switch for data/PCs, and another dumb switch for SIP phones exclusively. That works too, but is inelegant and can lead to lots of cable runs.


Phones:


Read some reviews, and get good SIP phones for your office. Sound quality differs substantially between models (mostly due to different microphones &amp; speakers).
A SIP phone is basically a micro-computer. It needs an IP address (DHCP), it must be registered with Asterix, from time to time you might want to update the phone firmware, etc. Plan how you will administrate this.


Asterisk for Small Business - Where to Start?
Are there some living things where the distinction between plant and animal is blurry or unclear?  I wouldn't know where to put something like yeast.  I also thought of photosynthesis, but then I heard that some salamanders have photosynthetic cells.  So, what's the difference between a plant and an animal, formally?  
