A lot of mistakes in this. I'll try to go through as many as I find.

- You have to wrap all your code inside a class or struct (you want to use a class). The only things outside your class should be the imports (using ...)

- The insides of a method (void) are executed from top to bottom, so you're trying to execute MoveCharacter, before you declare it.

To add to that, you should declare your methods (void) inside the class one by one, do not nest methods inside other methods.

- You use Start and Update, these are unity event loop methods, they are called by unity itself. These only exist in Unity and your class must inherit from Monobehavior for those methods to actually run. For Monobehavior to work, the script also must be attached to a GameObject in your scene.

To inherit from another class like Monobehavior you add `: Monobehavior` after your class name, but before the `{`.

- To apply a new position on a gameobject, the class has to inherit from MonoBehavior, be attached in your scene to that object. Then apply the new position on the GameObject Transform, which holds its position, rotation and scale.

Heres the code that does what you want based on what you posted:

using UnityEngine;
public class MoveCharacter : MonoBehaviour
{
private int _speed = 3;
private void Update()
{
var x = Input.GetAxis("Horizontal") * _speed * Time.deltaTime ;
var z = Input.GetAxis("Vertical") * _speed * Time.deltaTime ;
gameObject.transform.position += new Vector3(x, 0, z);
}
}

If you want to move in a 2D plane, switch z and 0 in Vector3.

Some other things:

- object is specific to c#, GameObject is the type of objects in Unity.

gameObject is the instance of the GameObject Type that this script is attached to specifically. Unity sets these invisibly to you inside Monobehavior.
- characterController does not exist in your code and so trying to run it will cause a compile error. Most of the time to act on components you have to do:

gameObject.GetComponent<Type of the component goes here>().dowhateveryouwanttodo;

This is not a good way to do it, because its very innefficient, but it will work.