Simulation is a far too general term. What kind of simulation? What kind of system do you want to simulate? If you're talking about a dynamic system (i.e., differential equation of motion), then at the base all you need is some good numerical integrators library, then probably some linear algebra library, and then you just implement the functions that compute the time-derivative of the state vector and feed it to an integrator, that's it. If it's a more complex system, you might need quite a bit of code to model the dynamics of that system (instead of manually writing out the complete differential equations).
There are certainly simulation tools written in C++, but these are very complex, and specialized, bits of software - not something you just go out and write in a day or three. As Mike_2k said, what kind of system do you want to simulate? If you can get into some specifics, we may be better able to suggest some tools to try.
thank you all , i want to simulate the e- commerce starting from the autheticatin server in which the website and the bank are registered then the simulating the web site that contains the itemes to be sold and the payment method until the whole procedure ends.
this is just a start after that i want to add some risk analysis ,decisssion rules and prediction .