Designing your own GUI system is very complicated and difficult if you're a beginner to MonoGame. SpriteBatch will generally be used to render all the graphics for your game, including your GUI stuff. You can pass things like image, source rectangle, destination rectangle, etc to SpriteBatch.Draw, and have it render to the screen.
So you'd build your GUI around SpriteBatch.Draw, keeping track of the various GUI element's positions, sizes, images, and so on. Alternatively, using an existing UI framework, like the one in monogame extended, would be much easier.