The way server is now sold is much more complex than recently. See this thread: http://help.filemaker.com/app/answers/detail/a_id/13102
You'll also need to research the recommended system requirements (see the knowledgebase link above) and make sure that you have a computer to use as the server that meets those requirements.
Ultimately, you may want to call a sales rep at FileMaker and ask questions as the number of simultaneous users that you need to support and whether there is a volume licensing package for FileMaker clients that fits your user base (and if you need it) will play a big role in your bottom line cost for upgrading.
Note that server 13 can support Fmp 12 clients.
The good news is that you should be able to simply upload your existing file to server and start using it with no modification needed (if you have been using FileMaker Pro to host your file(s). IF not, you still will likely need make little or no changes.)
You will need one copy of FM server plus each individual using the shared files will each need a licensed copy . . . . plus the price computer to run FM server on. You can run FMServer on a computer that has other duties, but it is strongly recommended you have a dedicated computer.
Filemaker does offer bundles (example 5 copies of FMP plus one copy of server) at a discounted price. Time to time they offer other specials, but I do not know what is available now.
As PhilModJunk said above, a Filemaker sales rep can help you out the best.