On the top panel, visit System -> Administration -> Synaptic Package Manager.
Search for "spamassassin".
Install "spamassassin" and any dependencies it requires.
- Launch Evolution.
Go to Edit -> Preferences -> Mail Preferences -> Junk.
Enable at least the first option box, "Check incoming mail for junk."
Go to Mail Accounts, select your email account, and click [Edit].
Click the Receiving Options tab, and select "Check New Messages For Junk Content".
Spamassassin itself requires no setup in most cases. If you really wanted to know all that, open the Gnome Help browser (System -> Help), type Ctrl-l (for "Location), and in the box type "man:spamassassin". Alternatively, you could open a terminal (Applications -> Accessories -> Terminal) and type "man spamassassin". There is additional documentation in "/usr/share/doc/spamassassin". (Places -> Home Folder, type Ctrl-l (Location), then enter "/usr/share/doc/spamassassin".)