Simply speak into your phone or computer while recording a greeting with your business phone service. If you need to create your own audio file, the voice memo app on your phone should do fine.
Most of you will agree when I say that one of the coolest features of Google+ is Circles, which lets you create groups of people to share specific things with, rather than sharing with everyone in your network by default. Today I came across an iOS app that basically has the same system in place, but for voicemail. It’s called On Voicefeed and it lets you build groups (family, friends, work, girlfriend/boyfriend) and set personalized voicemail greetings for each of those groups.

Keep in mind that short greetings are often much better then trying to say way too much that could confuse your callers. Some pointers to ask, “please leave your name and number”, or maybe ask them to go your business website to contact you by email, or even as simple as text.
Voicemails need to maintain a professional consistency that’s aligned with the entity it’s representing. That said, the structure can vary depending on the situation. There’s no template set in stone. In fact, trite and generic should be off the table. The goal should be a balance of uniqueness and practicality.