GetComponent help needed CSharp

hey everyone, GetComponent is somethign that i never realy understud and have decided to use send message instead(i truely dont knwo which one is better tho i believe its the GetCOmponenet) and right now i thought i might try and use it for the sake of learning. here is waht i got:

public nexus componentNexus;

void Start () {
		
		componentNexus = GetComponent(nexus);
	}

(its a long script so it messed up when i took out the parts that had nothing to do with this so you dont have to read much)

anyway a bunch of errors apear and i have tried everyting i could and nothing seems to work. pls some one help me. ty very much everyone

C# Example of how to use GetComponent.

CharacterController controller;

void Start()
{
    controller = GetComponent<CharacterController>();
    controller.enabled = true;
}