I'm absolutely useless with JavaScript, but I need this table to work with it.

I'm trying to create a table that will populate a row of cells based upon a selection from a drop down menu. The drop down menu sits at the top of the third and final column in my table and should populate the cells directly below it. Each selection from the drop down menu has to populate 23 cells.

Finally the page is written in HTML5 with an external CSS3 stylesheet and it will have an external JavaScript file to make the whole thing work.

If anyone can tell me how to do it, even if it's just the basic workings of the code, it would be appreciated.

Here is the code for the table:
Code:
<form id="alive-chat">       
            <table>
                <thead>
                    <tr>
                        <th>
                            <span class="bgthead" >Vendor</span>
                        </th>
                        <th>
                            <span class="bgthead" >Product</span>
                        </th>
                        <th>
                        	<span class="bgthead" >
                                <select name="features">
                                    <option value="1">Operators Included </option>
                                    <option value="2">Additional Operators </option>
                                    <option value="3">Mobile Chat </option>
                                    <option value="4">Visitor Chat Window </option>
                                    <option value="5">Multiple Website </option>
                                </select>
                            </span>
                        </th>
                    </tr>
                </thead>
                <tbody>
                    <tr>
                        <td>
                        <span class="bg" >Alive Chat</span>
                        </td>
                        <td>Lite</td>
                        <td>2</td>
                    </tr>
                    <tr class="alt">
                        <td>
                            <span class="bg" >Alive Chat</span>
                        </td>
                        <td>Pro</td>
                        <td>2</td>
                    </tr>
                    <tr>
                        <td>
                            <span class="bg" >Alive Chat</span>
                        </td>
                        <td>Pro+</td>
                        <td>2</td>
                    </tr>
                    <tr class="alt">
                        <td>
                            <span class="bg" >Zopim</span>
                        </td>
                        <td>Lite</td>
                        <td>1</td>
                    </tr>
                    <tr>
                       <td>
                           <span class="bg" >Zopim </span>
                       </td>
                       <td>Basic</td>
                       <td>1</td>
                    </tr>
                    <tr class="alt">
                       <td>
                           <span class="bg" >Zopim </span>
                       </td>
                       <td>Advanced</td>
                       <td>1</td>
                    </tr>
                    <tr>
                       <td>
                           <span class="bg" >Velaro</span>
                       </td>
                       <td>Basic</td>
                       <td>&nbsp;</td>
                    </tr>
                    <tr class="alt">
                       <td>
                           <span class="bg" >Velaro</span>
                       </td>
                       <td>Business</td>
                       <td>&nbsp;</td>
                    </tr>
                    <tr>
                       <td>
                           <span class="bg" >Velaro</span>
                       </td>
                       <td>Omni</td>
                       <td>&nbsp;</td>
                    </tr>
                    <tr class="alt">
                       <td>
                           <span class="bg" >Comm 100</span>
                       </td>
                       <td>Basic</td>
                       <td>2</td>
                    </tr>
                    <tr>
                       <td>
                           <span class="bg" >Comm 100</span>
                       </td>
                       <td>Professional</td>
                       <td>2</td>
                    </tr>
                    <tr class="alt">
                       <td>
                           <span class="bg" >Comm 100</span>
                       </td>
                       <td>Enterprise</td>
                       <td>2</td>
                    </tr>
                    <tr>
                       <td>
                           <span class="bg" >Live Chat</span>
                       </td>
                       <td>Solo</td>
                       <td>1</td>
                    </tr>
                    <tr class="alt">
                       <td>
                           <span class="bg" >Live Chat</span>
                       </td>
                       <td>Team</td>
                       <td>1</td>
                    </tr>
                  <tr>
                       <td>
                           <span class="bg" >Live Chat</span>
                       </td>
                       <td>Enterprise</td>
                       <td>1</td>
                    </tr>
                    <tr class="alt">
                       <td>
                           <span class="bg" >Live Zilla</span>
                       </td>
                      <td>One</td>
                      <td>&nbsp;</td>
                    </tr>
                  <tr>
                      <td>
                          <span class="bg" >Live Zilla</span>
                      </td>
                      <td>Pro</td>
                      <td>&nbsp;</td>
                    </tr>
                    <tr class="alt">
                      <td>
                          <span class="bg" >Olark</span>
                      </td>
                      <td>Bronze</td>
                      <td>1</td>
                    </tr>
                    <tr>
                      <td>
                          <span class="bg" >Olark</span>
                      </td>
                      <td>Gold</td>
                      <td>4</td>
                    </tr>
                    <tr class="alt">
                      <td>
                          <span class="bg" >Olark</span>
                      </td>
                      <td>Platinum</td>
                      <td>8</td>
                    </tr>
                    <tr>
                      <td>
                          <span class="bg" >Olark</span>
                      </td>
                      <td>Ultimate</td>
                      <td>15</td>
                    </tr>
                    <tr class="alt">
                      <td>
                          <span class="bg" >Whos On</span>
                      </td>
                      <td>&nbsp;</td>
                      <td>&nbsp;</td>
                    </tr>
                    <tr>
                      <td>
                          <span class="bg" >Live Help Now</span>
                      </td>
                      <td>&nbsp;</td>
                      <td>1</td>
                    </tr>
                </tbody>
            </table>
        </form>