Easy method: apt-get remove ssh ftp telnet apache (etc.). Alternatively, use the update-inetd command, e.g. usr/sbin/update-inetd --disable telnet. Services you may want to consider disabling include echo, chargen, discard, daytime, time, talk, ntalk and r-services.
The best documentation for Debian is available at http://www.debian.org/doc
- possibly the Developers Reference manual might be your best bet amongst these. Also, have a look at the harden-doc package, and install the harden, harden-clients and/or the harden-system package.
When it comes to security, a lot of reading is generally required