iirc nagios was mrtg/lrrd?
ok. first you'll need a running snmpd (it's the server) and it's configured (depends on the snmpd) maybe in /etc/snmpd.conf. have a look into that file. things like diskspace usually have to be enabled there.
as you say you allready get your uptime i asume a snmpd is up and running.
the layout of snmp first looks a bit scary, but if you're once familiar with it it's not half that wild. please don't ask me why it's called "simple network manaegment protocol" (or so?).
to get values from the host you'd like to watch use snmpwalk (man snmpwalk):
$ snmpwalk -v 1 -c public localhost system
"system" should give you quite a lot of output, to stop it: crtl^c
now you could use a more specific string to query. for example disk or memory usage:
$ snmpwalk -v 1 -c public localhost disk
$ snmpwalk -v 1 -c public localhost mem
to actually know what string means what see if there are *-MIB.txt on your system. they describe these short strings better for different interfaces/devices.
$ locate MIB
here's a quite useful link i just googled:
ask, if you'd like more info.