The default file name given to your solution will match the project name.  I.e. a SharePoint project SharePointProject1 will result in a package SharePointProject1.wsp.

If you want VS to spit out a different name, modify the name attribute in the Package\Package.package file for your project.

To open the file mentioned in the above post, use Notepad as Visual Studio will automatically detect the .package extension and open the associatied template file and NOT the .package file itself.

Also be sure to remove the write protection from the file otherwise Notepad cannot overwrite it.