I am trying to make my website mobile friendly and am new to programming.

I thought that I would use a simple forms drop down menu for now and program a proper mobile menu later.
The menu isn't working properly. The selected option shows in the box and the screen refreshes but it doesn't change pages.

An example of the code that I use is:

HTML Code:
<div id="drop-down">
      <form name="form">
         <SELECT name="guidelinks" onChange="window.location=document.form.guidelinks.options[document.form.guidelinks.selectedIndex].value">
    <option selected>Main Menu
    </option>
    <option value="index.html">Welcome</a></li>
    <option value="author.html">About the author</a></li>
    </select>
    </form>
   </div>
What do I need to do to get the list to work properly?
Thanks in advance.