Add an OPTION tag like this between your SELECT tag and first OPTION tag:
<option selected disabled>↓Pick one↓</option>
The "selected" attribute means it will be selected by default when the page loads, and the "disabled" attribute means it cannot be selected when the buyer clicks open the menu. The part after that you can customize if you wish to determine what's shown for that item in the menu (the ↓ I provided in my example above adds a down-arrow symbol :^).