Please be more clear in what you're trying to do.
If you are trying to press a button in your own program, you can call the command button's Click function.
If you are trying to press a button in another program, you will have to use something like SendKeys.
Or, the better way, use the Windows API functions like FindWindow to find the handle of the parent windows until you get the handle of the button. Then you can use something like SendMessage with the correct command (WM_LBUTTONDOWN and then WM_LBUTTONUP)
To run the program initially, you can use Shell, ShellExecute, or ShellExecuteEx depending on your needs.