How to save data into SQL server?

div class="row">
    <div class="col-sm-4">


        @using (Html.BeginForm("Save", "return", FormMethod.Post, new { id = "popupForm" }))
        {
            <div>
                <h1>CAR RENTAL</h1>
            </div>


            <div class="form-group">
                <label class="form-label">CAR NUMBER</label>
                <input type="text" id="carnum" name="carnum" class="form-control" placeholder="Enter Car Number" required>
            </div>

            <div class="form-group">
                <label class="form-label">CUSTOMER ID</label>
                <input type="text" id="custid" name="custid" class="form-control" placeholder="Enter Customer ID" required>
            </div>

            <div class="form-group">
                <label class="form-label">Date</label>
                <input type="text" id="rdate" name="rdate" class="form-control" placeholder="Date" required>
            </div>

            <div class="form-group">
                <label class="form-label">Days Elapsed</label>
                <input type="text" id="elapsed" name="elapsed" class="form-control" placeholder="Enter Days Elapsed" required>
            </div>

            <div class="form-group">
                <label class="form-label">FINE</label>
                <input type="text" id="fine" name="fine" class="form-control" placeholder="Enter Fine" required>
            </div>

            <div class="form-group">
                <input type="submit" value="Save" class="btn btn-success" />
            </div>

        }

    </div>
</div>

public ActionResult Save(returncar returncar)
        {

                if (ModelState.IsValid)
                {
                    db.returncars.Add(returncar);

                    var car = db.carregs.SingleOrDefault(e => e.carnum == returncar.carnum);
                    if (car == null)
                        return HttpNotFound("Car number not found");

                    car.availability = "Yes";
                    db.Entry(car).State = EntityState.Modified;
                    db.SaveChanges();
                    return RedirectToAction("Index");
                }
            return View("~/Views/Return/Index.cshtml", returncar);
        }