roupe93 is talking about the FPS Creator application available at
http://www.fpscreator.com/. I played around with it three years ago and it's really easy to use and import your own models. There is actually a video tutorial somewhere which shows you how to convert models to a format understandable by the engine via a Command Line Interface.
If your to lazy and don't want to create your own models you can use the ones provided with the engine and you can also download model packs. Once you've finished developing the game your even allowed to sell it.
It is also handy to know the programming language FPI Script, a language developed by the developers of FPS Creator. It's not to difficult to learn either I believe most of its a predetermined set of states used with simple if/then syntax.
FPS Creator was developed using "DarkBASIC Professional", which was created by those who created FPS Creator using the Basic language.