You don't need to defag your hard disk - performance losses due to fragmentation are minimal. This is one of the up-sides of some filesystem design decisions. There are of course down sides as well. For example searching through the names of all the files on the system (aka walking the directory tree) takes longer on unix-like systems.
Ubuntu, and most other main stream Linux distributions come pre-configured with recurring tasks which clean up the temporary directories. FYI these directories are /tmp and /var/tmp. They have slightly different uses - for example, /tmp is cleaned every reboot, and /var/tmp is not. The details aren't so important for most people, so if you don't care about it, no worries.
If you're looking for some fellow Linux users to chat with, search for a local Linux User Group (LUG). There are LUGs all over the place, and they often hold regular meetings, talks and social events as well ask communicating via mailing lists and so on. Looks like this is the closest thing to you: http://www.humbug.org.au/