The best way to go about doing what you want is to create a group called "www" or "web", then chgrp the files and directory to be in that group, and change the permissions of the files to be read and writable by the group using chmod. Then later you can add anyone to the www group, and they will be able to modify the web files.
Of course, if you just want the quick and dirty way, a "chown -R username.username /var/www" should do what you want.
Background: Unix files belong to an owner and a group. The files can be specified to be readable, writeable and/or executable for the owner, the group, and everyone else. You can view all this by doing an "ls -l" which will produce output similar to:
-rw-r--r-- 1 root root 364 May 13 2002 index.html
The first character of that colum specifies if it's a directory, the next character if it's readable by the owner, the next if it's writable by the owner, the next if it's executable by the owner. Then it starts over for the group, then it starts over for everyone else.
These permissions are modified by the chmod command, which is usualy used by translating the numeric values to binary and setting those bits, thus "chmod 777 index.html" would change the output to:
-rwxrwxrwx 1 root root 364 May 13 2002 index.html
A simpler way to do this is a+r for "all plus read" and so on. See the man page for more information.
Hopefully that's enough to get you started.
Matthew