Suitable SNMP software and instructions should be available from the vendor of your Linux distribution.
SNMPD from the Net-SNMP project is very commonly used, see: net-snmp.sourceforge.net/
There is a good tutorial that explains the installation of the NET-SNMP deamon on a Debian Linux system on the DebianHelp website:
www.debianhelp.co.uk/snmp.htm
After installing and configuring this daemon you should be able to monitor vital system data (e.g. the CPU load) using PRTG Traffic Grapher and IPCheck Server Monitor. Simply create a new sensor, choose "SNMP Library Sensor" and select the "Basic Linux Library" from the list of available libraries.
See the links below for instructions how to monitor Linux servers.