adding new C++ code with a DLL in an exe?

[Mattish]

Ok sorry for being so newby im trying to learn what i can in my spare time which is not a lot of time.

ok so, i have a game server for a game (Knight Online) which is an exe, i would like to add extra functions into such exe but i do not have the source.

Where would i start if i wanted to inject a DLL file, with new functions into that exe?

the thing im confused about is the actual start of the dll, would i just do a simple

#include <xx>
#include <xx>

using namespace std;

int main(){

codehere

}

im just confused on how to actually get the code to be implemented by a dll.


sorry if its a terrible question but im so confused :P

all help applrecitated