Assuming they are married, address them first off as Mr/Mrs x, which will hopefully get the response of "Oh, call me **first name**", unless you are young teen; they might want to keep it a bit more formal then.
Take a small gift; flowers (if going to their house and not out somewhere for a meal, too awkward to carry), chocolates, nice biscuits.
Get some background from your girlfriend about their jobs, hobbies, holidays. You can then use that in conversation as you need to be asking questions as well as talking about yourself.
If there's coffee or a meal, offer to help tidy up or do the washing up.