Quote: "how do i make gui for a 3d game its really confusing me."
That question is equivalent to asking
how do i make a game. The GUI is the game and there are a bazillion ways to code one. Understanding the primary/secondary operation behind a GUI, will help you write the code you need.
I've written several GUIs and in my opinion the primary operation is based on Device Input (Mouse) to control a
Pointer, Collision Objects (2D/3D Clickable Shape) for
Gizmos, and Collision Detection between the Pointer and Gizmos. Display Objects (Sprites, Meshes, Images) are used represent Pointers and Gizmos visually and they are secondary for operation.
So before you go out and tackle a GUI, you have to understand how to manage mouse input, create a clickable areas, and detect collision between the two, and render display objects that represent pointers/gizmos.
Do you have these basics?