The items under the home group have been ordered by the product, apart from any extensions which have been added. Reordering them is not something which by default belongs to the available options.
However, adding your own group in a specified order rather then at the end is possible. For this you can use the insertbefore
attribute in your ext:extension
element, roughly like so:
<ext:ribbontoolbars>
<ext:add>
<ext:extension assignid="MyGroupID" name="My Name" pageid="HomePage" insertbefore="PublishGroup">
...
</ext:extension>
</ext:add>
</ext:extension>
See also my article on Tridion Developer which explains how to add a Ribbon Item Group.
What can help you for all the available attributes is to refer to the schema of the extension configuration. There are a couple of them and you can find them on your CM server in the ..\Tridion\web\WebUI\Core\Schemas
directory.
To find the IDs which you can use in the insertbefore attribute, you basically need to inspect the HTML of the UI. You will see that every div
which makes up a group will have an ID, and its those ID names you can use. Which in turn allows you to basically place your extension group, before any existing item.