On the top panel, visit System -> Administration -> Synaptic Package Manager.
Search for "spamassassin".
Install "spamassassin" and any dependencies it requires.
- Or write in terminal
apt-get install spamassassin
- Launch Evolution.
Go to Edit -> Preferences -> Mail Preferences -> Junk.
Enable at least the first option box, "Check incoming mail for junk."
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".)