Check selected value in a DropDownList with entity

I'm a beginner in ASP.NET and I have a relatively simple question ...

How to get the selected value in a dropdownlost like this and pass it to a variable within the same cs.html to do a

<div class="form-group">
        <label class="control-label col-xs-2" for="ClassificacaoId">Classificacao</label>
        <div class="col-md-10">
            @Html.DropDownList("ClassificacaoId", null, htmlAttributes: new { @class = "form-control" })
            @Html.ValidationMessageFor(model => model.ClassificacaoId, "", new { @class = "text-danger" })
        </div>
    </div>

For example, I select the field requests, a specific field must appear.

I would like to do an if identifies who I picked up in DropDownList, how do I do that?