To get snmp monitoring happening you need a couple items:

Generally, I apt-get install snmpd on the machine being monitored. The simplest config file for snmpd is the following line:

rocommunity public

Which provides read-only access to the public community to anyone who can physically reach the device. Restart snmpd any time you change the config file.

I like to also apt-get install snmp scli on the monitor to provide some low-level tools for troubleshooting.

At this point, I would confirm connectivity by signing into the monitor and running snmpwalk hostname.of.monitored.machine -c public -v 1. This should dump out gobs of information. You can also use scli hostname.of.monitored.machine and then run show or monitor in the shell-like interface.

I would recommend against leaving the device being monitored wide open because someone could theoretically do something bad to you. Lock down access through firewall rules or through snmpd configuration - you can always retest from the monitor to make sure that you still have access.

Getting cacti running is simply a matter of pointing it at the device and letting it probe. You can add other queries once the initial connection is made (i.e. query CPU or disk) to get more information. The documentation within cacti is pretty good - RTFM.