What you need
A Mac running Mac OS X 10.2. or later, any hardware drivers needed for your USB device, the Sound pane in System Preferences, Audio MIDI Setup (bundled application, in /Applications/Utilities), audio applications as desired.
Potential problems
Any USB device can result in driver or hardware problems. Although drivers for professional devices may have extensive controls in their ASIO supporting software, play through and similar features may not be controllable on cheaper models. You could find desired features implemented in third party software, such as USB Audio's shareware replacement driver for some devices. Digital signal processing of audio is liable to result in distortion and noise on occasion: feedback between output and input can be troublesome, but sometimes noise can develop and become amplified internally, requiring resetting of processing by turning software off and back on again.
Further info
The third-party USB Audio driver is at www.usb-audio.com, Audio Units information and links at www.audio-units.com, and general OS X audio information is at www.osxaudio.com.
Checklist
Install your device and any software according to its manual.
Check its presence using Apple System Profiler.
Set it for input in the Sound pane.
Configure and tune settings using Audio MIDI Setup utility.
Check and configure frequency responses and applications
Click 'Next Page' below for the step-by-step guide...Installing and configuring USB sound devices