To have a flexable 'tip of the day' feature I would suggest using a database. it would be good to have the database pre-filled with lots of tips and either select one randomly or loop through them incrementally.
You will need to create a table with the following fields:
TipID - int (unique, Not Null, Auto Increment)
Tip - Text (Not Null)
ShowDate - Date (Not Null) - only if you wish to assign each tip to a date.
Then Create an administration form to add the data to the database, You will need a text box for ShowDate (if required) and a textarea for Tip.
Finally depending on the method chosen (random, loop, specific date) create the page to display the tips. I would suggest just keep you table size open - that way you do not need to specify the size.
Basically you will be mainly writing SQL queries to Add or Display data from the Database.
Check out www.mysql.com for help.
Check out www.php.net for help with PHP.