When you install Magento the admin access directory is set to site_admin. This makes it very easy for a hacker to find the admin access area.
To change this to make it more difficult navigate to /app/etc using your favorite ftp program or your access panel.
Then download local.xml.
Open local.xml in a text editor such as textpad, wordpad or even notepad.
Look for the following usually around line 53
<adminhtml>
<args>
<frontName><![CDATA[site_admin]]></frontName>
</args>
</adminhtml>
Change site_admin to whatever you need.
Note** Before you upload it login to the admin panel and disable the cache. You can find this under system/cache management.
Or using ftp delete everything in /var/cache
Re upload the changed local.xml. file to /app/etc.
Log back in using your new directory name and re enable the cache. You can find this under system/cache management. If you deleted everything in the the var/cache folder you can skip this step.
That’s it.