Check Software Sources, i.e. Go to System->Administration->Sofware Sources->mark restricted (proprietary software) and multiuniverse as well.
Then from Ubuntu Software Center find Skype->Install.
Otherwise, there is a good Ubuntu guide: http://ubuntuguide.org/wiki/Ubuntu:Natty
Here is about Skype:
* Install pre-requisites:
sudo apt-get install libqt4-dbus libqt4-network libqt4-xml
* To get the most recent version, download and install the 32-bit version:
* In the past. some users have noted that they cannot get their microphone inputs to work with any version later than 220.127.116.11. They installed: