You don't really need an adapter if your speakers are self-powered. But if you have a set of unpowered speakers like the type used with home theater/home stereo setups, then you need to plug the iMac into an amplifier of some kind.
The iMac has several ways to run speakers.
The iMac has the traditional 3.5mm audio out jack (headphone jack) so you can plug in thousands of inexpensive standard self-powered speakers made in the last 30 years or so. That's how I connected my speakers. I just bought a cheap common 2 speakers + subwoofer set at a generic PC store and plugged it into my Mac. You can also use the 3.5mm audio out jack to connect to an amplifier or receiver that can power speakers. You don't need an adapter this way, just a very low priced common audio cable with the correct connectors at both ends.
The iMac has USB ports, but it is not common for consumers to drive speakers through them. If you have seen listings for USB audio adapters or interfaces, those are typically for audio professionals who need to connect their Mac to USB audio equipment used in recording and mixing music.
The iMac has Bluetooth and Wifi, so if you have one of the increasingly popular Bluetooth speakers, you can send music wirelessly from the iMac to the Bluetooth speaker. Many new audio receivers can take a Bluetooth or Wifi (AirPlay) audio signal from your iMac and send it through a home theater speaker system.
