We'll warn you now in case you haven't been warned yet. Do not make changes to the default template set...see here for details

Now that you've been warned not to do that, make sure you have a custom template set in your site that you can make changes to. Many times this is taken care of by the installer but you can easily create one if you haven't yet. Here are the steps:

1. Check the DESIGN > TEMPLATE SETS admin tool to make sure you have a template set besides default template set you can use that template set for your changes and you do not need to carry on
2. If you don't see a custom template set to contain your template/html design customization you only need to click the DESIGN > TEMPLATE SETS > [CREATE MAIN TEMPLATE SET] button
3. Name that new template set the way you want in the popup box that appears after clicking the [CREATE MAIN TEMPLATE SET] button. Make sure there are no empty spaces in that template set name as your new template set directory will be the name you type here and directory names can't have spaces.
4. Once done click the [CREATE MAIN TEMPLATE SET] button at the bottom of that popup. Or click cancel to stop the process.
5. That will create a template set of the name you just inserted. That new template set is a working copy of the default template set that you can make modifications to for your design needs. Next make your new template set active in the above admin tool page by making sure the "active" checkbox is checked for that new template set.
6. Next click the checkbox in the "admin editing" column if you are ready to begin editing your client side design.
7. Click the save button to save these last two changes.

Now you have a custom template set that will contain your HTML/template customizations.