Dropdownlist with MVC4

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MVC

C#5.0

MVC4

I am new in MVC and i need to fill database values to dropdownlist and while save i need to save dropdownselected item to database.
My code

View

<div class="editor-label">
                @Html.LabelFor(model => model.TaxID)
            </div>
            <div class="editor-field">
                @Html.DropDownList("Tax", "select Tax class")
                @Html.ValidationMessageFor(model => model.TaxID)
            </div>

controller for loading values from database to Dropdown

ViewBag.Tax = new SelectList(db.SHP_Taxes, "TaxID", "TaxName");

for Save in controller

[HttpPost]
        public ActionResult NewProduct(Product newProduct)
        {
                        if (ModelState.IsValid)
            {
                try
                {
                    SHP_Product pro = new SHP_Product();
                    var max = db.SHP_Products.Select(x => x.ProductID).Max();
                    int inter = Convert.ToInt32(max);
pro.TaxID = newProduct.TaxID;
  }
                catch (MembershipCreateUserException e)
                {


                }
            }
            ModelState.AddModelError("", "error occured");
            // If we got this far, something failed, redisplay form
            return View(newProduct);
        }

in controller model.taxid is null