Set UVs depending on input

I've made a function, to create basically a face for the a mesh, that i'm building through scripting. Altho, vertice, triangles and normals is all setup and calculated, UV's is not.

I honestly, have no idea how to setup the UV's, neither how it corresponds to what is what (An explanation too would be appreciated)

This is my create face code. It takes "Input" (A position, width and height) and then changes this, depending on that. I would think, that how it works?

Code:

 private void CreateFace(Vector3 position,int width, int height) {
     Mesh_Vertices.Add(new Vector3(position.x, position.y, 0));
     Mesh_Vertices.Add(new Vector3(position.x+width, position.y, 0));
     Mesh_Vertices.Add(new Vector3(position.x, position.y+height, 0));
     Mesh_Vertices.Add(new Vector3(position.x+width, position.y+height, 0));
 
     int a = Mesh_Vertices.Count-1;
     int b = a-1;
     int c = a-2;
     int d = a-3;
 
     Mesh_Triangles.Add(a);
     Mesh_Triangles.Add(b);
     Mesh_Triangles.Add(d);
 
     Mesh_Triangles.Add(d);
     Mesh_Triangles.Add(c);
     Mesh_Triangles.Add(a);
 
     //This isn't correctly setup.
     Mesh_Uvs.Add(new Vector2(0,0));
     Mesh_Uvs.Add(new Vector2(1,0));
     Mesh_Uvs.Add(new Vector2(0,1));
     Mesh_Uvs.Add(new Vector2(1,1));
 }