Launch FileMaker Pro 11.
Select Open from the file menu and select your FileMaker 6 files. (select all the files that make up your invoicing database to open them all at once.)
FileMaker will then created converted copies of all your selected FileMaker 6 files.
A conversion from FileMaker 6 to 11 will not be perfect. Once you can open the files, you'll probably need to make some additional changes to scripts, password settings and possibly some relationships to get everything working perfectly.
Here's a knowledge base article on the conversion process that you should read: http://help.filemaker.com/app/answers/detail/a_id/750/kw/converting