If you are setting something up that will be used only within one restaurant while the waiters are connected to WiFi, you could use a PC on premises. If they need to be able to access the data from more than just within the building connected to WiFi, you'll need to have web hosting account with mysql support.
What you'll probably need to do is to set up a website (register the domain), and find a web hosting provider that has MYSQL built in. Then you'll want to use the PHP script (which you can find by searching the forums) to connect to the MYSQL database.
Either way, the simplest approach is to set up a webhosting account that supports PHP and MYSQL. Most hosting accounts have PHPMYSQL built in for administration. If you are advanced with SQL, I'd recommend Oracle's MYSQL workbench for working with the database.