How can I identify a device on my network?

Can you explain how I can go about identifying a device on my network? I'm having trouble pinpointing the source of some unusual activity and I'm wondering if there's a specific process or tool I can use to pinpoint the device in question. Is there a certain set of steps I should follow, or is there a particular software or hardware solution that would be most effective for this purpose? Any guidance you can provide would be greatly appreciated.