It really depends on how much you want to spend and how good you want audio. You can get some pretty decent computer speakers that connect using a USB cable. Most of these are designed as plug-and-play for either Windows or Mac OS and do not need any additional software or drivers. Prices probably range from $50 to $200 (I'm not sure because I haven't researched this).
Nest step up is to get a really good sound card, such as Asus Essence STX II, Omega Claro Halo, or a EVGA NU AUDIO. These range from $250-500. Then you can get some really nice computer speakers or even ultra nice studio monitors. The computer speakers can be up to $500. The Studio Monitors range from $500 a pair up to $10,000 or more depending. A pair of Yamaha HS8 would be an excellent value for the money while providing excellent sound.
Lastly, if you want to do any recording or audio capture or instrument capture (like electric guitar), you really want an audio interface. This will take the place of a sound card. It's essentially an external ADC/DAC that is generally connected to your computer using a USB cable. These range from cheap Focusrite (about $150+) to the higher end stuff like RME ($750+). Then pair with your preferred studio monitors. The audio interfaces can have many inputs if you need and also balanced microphone inputs with phantom power.
It all depends on how far you want to go with this.
