The first step is to get a selection service, e.g. from any view or editor like this:
ISelectionService service = getSite().getWorkbenchWindow()
.getSelectionService();
Or, as VonC wrote, you could get it via the PlatformUI, if you are neither in a view or an editor.
Then, get the selection for the Package Explorer and cast it to an IStructuredSelection:
IStructuredSelection structured = (IStructuredSelection) service
.getSelection("org.eclipse.jdt.ui.PackageExplorer");
From that, you can get your selected IFile:
IFile file = (IFile) structured.getFirstElement();
Now to get the full path, you will have to get the location for the IFile:
IPath path = file.getLocation();
Which you then can finally use to get the real full path to your file (among other things):
System.out.println(path.toPortableString());
You can find more information on the selection service here: Using the Selection Service.