Adding a new class

Well, I'm wondering, how do you properly create a new Class object in the engine that can be used in other C++ files, initialized through the StandardMainLoop?

void StandardMainLoop::init()
{
   Platform::setMathControlStateKnown();
   .
   .
   .
   MyClass::create();
}

where MyClass is the new class... what do I need to have in my class' .cpp/.h files to make them work in multiple other files.