Just a few suggestions, but just be yourself, they have been in you're position before:
Call them sir and madam/ Mr. and Mrs. until they tell you otherwise
Shake the fathers hand if he offers it
Manners (don't use your phone at the table)
They may ask you what you are planning to do i the future so have an idea of what you are going to say
Casual convo i.e. no religion, politics, money etc.
Get there on time
Offer to split the bill (they are highly unlikely to let you, but take some money anyway, it shows that you are a gentleman)
Possibly flowers for the mum (may be a bit of a hassel in a restaurant)
All they want to know is that their little girl in happy and being well treated.You'll be fine just smile and be yourself. She obviously thinks you are ready to meet them.