I want add a combo box in notepad's client area,but I don't even know where to begin.

So could someone give me some ideas?

Thanks!

You want to look into code hooking, code caves, and the CreateWindowEx Win32 function. You will need to find a cave (an unused area in e.g. the code section of the .exe), write code there that calls CreateWindowEx to create the combo box, and place a hook at some location in the program that jumps to your cave. (of course don't forget to jump back to the original Notepad code after you're done in the cave)

If this explanation is too vague feel free to ask for more information, just don't expect someone to hold your hand and guide you through the process step by step . There are plenty tutorials out there that explain eg hooking.