Here is a brief list of some applications that are preinstalled with Xubuntu for your convenience. Find them in the menu on the left top corner of your screen. You can install more through the Ubuntu Software Center.
Internet
Firefox, web browser
Thunderbird, mail and news client
Pidgin, Internet messenger
Office
AbiWord, word processor
Gnumeric, spreadsheet editor
Media
gmusicbrowser, music collection software
Parole, media player
System & Settings
Ubuntu Software Center
Settings Manager