So you would start by creating an HTML form which would contain a file input:

@using (Html.BeginForm("Index", "Home", FormMethod.Post, new { enctype = "multipart/form-data" }))

{

    <input type="file" name="file" />

    <input type="submit" value="OK" />

}

and then you would have a controller to handle the upload:

public class HomeController : Controller

{

    // This action renders the form

    public ActionResult Index()

    {

        return View();

    }

    // This action handles the form POST and the upload

    [HttpPost]

    public ActionResult Index(HttpPostedFileBase file)

    {

        // Verify that the user selected a file

        if (file != null && file.ContentLength > 0)

        {

            // extract only the fielname

            var fileName = Path.GetFileName(file.FileName);

            // store the file inside ~/App_Data/uploads folder

            var path = Path.Combine(Server.MapPath("~/App_Data/uploads"), fileName);

            file.SaveAs(path);

        }

        // redirect back to the index action to show the form once again

        return RedirectToAction("Index");       

    }

}