You would need a monitoring solution installed on a server with network access to all of the servers you want to monitor, and depending on what you want to monitor and how you want to do it, you could need a user account on the operating systems of those machines (for example if you want to monitor using WMI (windows) or SSH (linux/unix)). This would not be necessary if you want to monitor using SNMP, but an SNMP agent would need to be running on the machines.
Other solutions would require a proprietary agent to be installed on the monitored systems.
There are free tools, and there are paid products which tend to be more feature reach and have better suppport.
If you are interested in free solutions, take a look at this question/answer: Free infrastructure/network monitoring software for SMB