Let me start by saying that English is perfectly fine.
In regards to invoice samples, I would suggest a simple Google search "sample invoice" or invoice templates", something along those lines, and that should find you some samples that you can get some ideas from.
An invoice needs to include transaction details of course, but in regards to design, you have an open book in regards to design. If you have access to a vector graphics program, then just a play around with some your own ideas and see what you can create.
While more limited in design features, word processing programs can also be used to develop your own invoice quite effectively.