System Preferences should be in your Applications folder. Open a Finder window, click Go in the top menu bar, then Applications. Click and drag the System Preferences from there back to your dock. Alternatively, you can always access System Preferences by clicking the Apple logo in the top left corner.
Acquire the newest version of Microsoft Office here: http://www.microsoft.com/mac
According to Microsoft's system requirements page Office 2011 will work on OSX 10.5, but you should definitely consider upgrading your operating system to 10.6 as Niel suggested.