How to do server side validation on the given c# code?

Question

I am a beginner and have no idea how to do server side validation on the form fields! Although I have done the client side validation by JavaScript, but server side validation require!

public ActionResult New()
{
return View();
}

//It is add method in controller

[HttpPost]
public ActionResult Create(string TeacherFname, string TeacherLname, DateTime HireDate)
{
Teacher NewTeacher = new Teacher();
NewTeacher.TeacherFname= TeacherFname;
NewTeacher.TeacherLname= TeacherLname;
NewTeacher.HireDate= HireDate;
NewTeacher.Salary= Salary;
TeacherDataController controller = new TeacherDataController();
controller.AddTeacher(NewTeacher);
return RedirectToAction("List");
}