VOIP Software is usualy provided pre-configured or with the necesary infromation from a Voip supplier's website.
One of the most popular is x-lite from www.counterpath.com. It is a 'free' version of their eyeBeam Voip product and has fewer features than the paid for product. If you are just 'testing the water' with Voip then x-lite will be fine, if you are planning to make intensive use of a voip system, but not likley to buy a dedicated Voip Phone or adaptor, then it may be worth investigating the paid for product with it's extra features and configuration options.
If you are going with the Skype system however, you are retricted to using software and adaptors made especialy for that system - although there is plenty of choice.