Describe a family meal?
Basically I don't know what to write about (and I don't want to make it feel awkward or too short) I want to write a scene where a girl is meeting her boyfriend's parents and they're having dinner in a large hall with lots of great food but I don't even know what they can do or say or eat...... I'm so stuck 
The girl is shy and awkward and the boy is sweet and confident and the family are all posh, stuck up and egotistical
Comments
Are we to see it from the girl's point of view?
First: She's shy and awkward. She wants to make a good impression on the parents, so she's probably dressed in her best. His parents are stuck up, egotistical, and posh. They'll probably have some prejudices against her, so they'll look for faults. She's probably aware of that, at least from the looks on their faces when they greet her. So how do they greet her? How do they look her over? How do they subtily send the message that she is inappropriately dressed and "not one of us, darling." How does she feel and how does she express it? If you've described each of them to yourself using Adrian's character list, you should be able to "see" and "hear" them. I assume she'll be more awkward, drop forks, hesitate before picking up the first one because she does not know which is the right one, try not to make a face when eating an unfamiliar food, etc.
The venue: Steven's answer's great. You'll need a fancy restaurant, even if the venue is a catered wedding at a private club, because you'll need the menu. Pick the city, search on Google Maps or the online yellow pages for restaurants. Each fancy one will probably have a website, for reservations, that will show what entrees they offer and what the decor looks like. You might want to type the entree name into your web browser to get the recipe (Plot point What if your heroine is allergic to an ingredient?)
I don't know many snooty rich people, so I don't know how fancy the place settings are. I know the huge Downton Abbey / Edwardian style in which all the silverware and glassware in the house is in front of the diner is passe; but you might want to browse through a ettiquette website or book that describes various place settings and the appropriate dining out manners. (Continental vs. American style of holding the fork is one example) Your rich, snooty parents might have good dining manners, or the girl might have better manners than they do.
I don't know where your story is set, but here's some good recommendations! Set your story in a specific place (that is real), then search in that area on Google Maps for fancy restaurants for them to dine in!
I have a feeling you mean a scene, as in a scene in a film. You can get them to sit down, order drinks, order food, have a conversation, eat, order, conversation (short), eat, order, eat, order and leave. Quite simple! Just plan it carefully - ensure readers would enjoy your story/film/scene.
If you're in the UK, a relative of mine runs a business that publishes books, magazines and plenty more. You can take a look at the website www.bylinepublishing.com but this is ONLY for UK clients.
I find writing dialogue for characters next to impossible until I have defined each of the characters involved in at least two, but for main characters three dimensions (I'll explain what I mean by that shortly).
Once I have done that and have a situation I find the characters tell me what they are going to say and the dialogue almost seems to write itself.
Define a character in three dimensions like this:
<Character name>
<Basic role in life / story and family position>
<Character> first dimension –surface personality, quirks, traits, visual appearance and behavioural choices
<Character> second dimension – product of backstory, belief systems, persona choices, decision and action drivers, parents and parents profession
INNER DEMONS (all protagonists should have one):
Examples: Cowardice, selfishness, addiction, fear, conceit, arrogance, hatred, resentment, bias, lack of confidence, stupidity, genius, heritage, poverty, ignorance, sociopathic insensitivity, naiveté, a spotty moral compass, sexual deviance… in general, any aspect of humanity that isn’t in line with the expectations of others or the accepted ground rules of success within the boundaries of your story.
<Character> third dimension – decisions, actions, behaviour, real world regardless of inner demons