Usage:
You can use spamassassin in many ways. But the lightest for the machine is by making use of the spamc/spamd combination.

First let's add a user for the spamd.

# useradd -s /usr/bin/false -b /nonexistent spamdaemon

Add this line to /etc/rc.local:

# /usr/local/bin/spamd -u spamdaemon -d -L -p 4321 -x

The final step: put this at the top of your .procmailrc. If you do not want to use procmail please check the online docs

:0fw
* < 256000
| spamc -p 4321 -f

:0e
{
EXITCODE=$?
}

:0:
* ^X-Spam-Status: Yes
caughtspam # This is the mailbox where all spam goes.

Customizing:
The spam filtering is not perfect. But since the feedback is very extensive you should be able to do what is necessary to get most spam filtered.

If you want to add custom settings to spamassassin you can create the file /etc/spamassassin.cf . All possible options are described in: man Mail::SpamAssassin::Conf

Much more info:
This is just a mini-HOWTO to get you up and running if you are in a hurry. Please go to spamassassin.org for further information.