I’m not sure if you mean sending keystrokes by as if you were typing in an in game console or something, or whether you mean just modifying content just as cheats. If you mean the first part then I have yet to find a way to do it. If you mean the second part then I’ll go over that now. First thing you need to do is learn how to find an open process and attach to it. From there you can modify the memory addresses (assuming you already have the addresses). If you don’t then finding the memory addresses can be tricky depending on the game. I would suggest learning to use cheat engine and Ollydbg. There are countless tutorials online for these. The easiest way to find an active process would be to use the FindWindow() function. So for example you could declare a variable and define it to the FindWindow():

HWND vWin =  FindWindow(null, ProcessName);

The first argument is the ClassName and the second is the WindowName. You can read more about this function on the msdn dev center. Let me know if you have anymore questions and I’ll try to help where I can. Good Luck!!

EDIT: forgot to mention, you should look up how to create a trainer in C++. This will help you get started in this project.