It would be obvious if you understood one thing:
You need to call the keybd_event function to press a key down, and it stays down until you call it again to release it with a KEYEVENTF_KEYUP. Now that you know how to simulate the up and down position of any key on the keyboard (the alt key is the WM_MENU), i don't forsee you having any problem using upper and lower-case letters, etc.