Echoing Skelly's link in the comments (and this comment too), I'd caution you that this just isn't how <select> lists are designed to work. A label will serve the same purpose, and is designed for this. If you do move ahead in this way, you're forced to start asking questions like this:

"user can select Please Select and submit that. How can i stop user from doing that?"

And now things are looking a little bit more hacky.

In any case, I think this is the wrong way to go about things, but here's how I would do it. First, I'd use a ViewModel (not ViewBag) and incorporate the <select> list into it. This way, you can use HTML helpers: which I believe is the more orthodox way to create a SelectList in MVC5.

public class ToolViewModel {
    public ToolDropdownList ToolDropdownList { get; set; }
    public string SelectedTool { get; set; }
    // ...
}

public class ToolDropdownList {
    public List<SelectListItem> Tools { get; set; }
}

Prior to the View being rendered using the ToolViewModel ViewModel, ToolDropdownList would have to be populated with SelectListItems. Each of these would represent a tool on the select list, with a Text, Value, and Selected attribute.

This is where you would add the Please select SelectListItem.

Then I'd render the view using HTML helpers and decorate the helpers with your Bootstrap classes.

<div class="form-group">
    @Html.LabelFor(m => m.SelectedTool, new { @class="text-primary" })
    @Html.DropDownListFor(m => m.SelectedTool,
        new SelectList(Model.ToolDropdownList.Tools, "Value", "Text"),
        new { @class = "form-control", @id = "sel1", @required = "required" })
</div>

If you still reeeeally wanted to allow a user to submit a form with Please select... selected, you could opt for some sort of special client or server-side validation rules to allow this.