Firstly, you will need to get the latest release of the mod tools, this contains all the source for the game, including all menus... then the next trick is to find the menu you want to change... this is a much bigger challenge than you might think...
The question I would ask is do you want to change the first menu, which you can but in the server code you have no control over any menu until you connect to the server.... if you want the change a menu in the game after connect then I can help you... send me a screen shot of the menu you are tyring to change and I will try to guide you on the development of new buttons... but I want to warn you it is not the cleanest coding task...
Thanks