Create a new sub-domain of your current domain, such as test.mydomain.com. When you're filling out the form to create the sub-domain it will ask you "Run this domain under the user:" select "create a new user" and fill out a username in the blank that appears after you make the selelection for "create a new user". This will create your sub-domain and attach it to the new ftp "user".
Wait until the clock icon next to the new sub-domain disappears when you refresh the "manage domains" page manage domain, usually 10-20 minutes, you also should get an email saying the new sub-domain is created and ready.
Next go to the "one click installs" page in the panel and pick Wordpress. Again when asked for database, you will be able to select "create new" to let the panel do the work for you.
When that process is complete you will get another email with the final install instructions.
If you put your new install on a separate sub-domain as instructed above, your existing website will be protected from your experiments.