As George said, follow a few of the tutorials on AJAX and MySQL... That is how I got started a couple years ago. I have done lots of database work with construct2 and it works really well. There are several forum members that will be able to help you out when you get stuck, but it is easier for us if you have specific questions, and sample code for us to work with.
If the restaurant already has a website, you can probably have that site host the database and app... rather than maintaining another service, but either way it should work fine.
setting up the tables and fields you want in the database is pretty straight forward, but getting the access required by AJAX and PHP working initially is the hardest part.