To make a new test account. This instruction sheet covers more than just one operating system, your system preferences may look a bit different but the way it is done remains the same.
This is the way it looks in 10.4, 10.5, and 10.6.
Open System Preferences/Accounts.
if the padlock is locked click on it and enter your password XXXXX unlock it.
Then click on the plus just above the lock.
Fill in the information as it is in the above image.
Make certain it says Administrator then click create account.
Now your new test account is fully functional and can be used to troubleshoot issues in your machine.
Depending upon what we are testing you will probably want to logout of your normal account and log in to this account.