I am also shy, and I never know what to say in social situations. I have also been accused of being snobby and stuck-up in the past, when really I'm just shy. The fact is, most people assume that when someone doesn't speak to them, they're snobby - it doesn't occur to them that the person might be shy. So you have to make an effort to smile at everyone, say hello, and try to make smalltalk. I realise this might be difficult for you!
I'm going to offer some practical advice based on my own experience, so I apologise if this is too detailed or isn't what you wanted - it's meant in the spirit of helpfulness! I know this might seem obvious to more outgoing people, but this is stuff that I actually had to try really hard with.
The smiling part is easy - smile as much as possible, and look happy and approachable. Imagine you're attractive and interesting and everyone would enjoy talking to you - hold your head up and look around you, don't shrink down into your shell. Make eye contact with people (don't put your head down!) say hello, and give a big smile. It's going to feel forced at first, but it gets easier with time.
When it comes to conversation, a shy person's best weapon is a few well prepared phrases. Ones that I use are:
"So how do you know X?" (X being the person who introduced us). Follow up by mentioning how you know X, and X then becomes a common topic of conversation.
"Isn't it warm in here/cold outside/windy/whatever?" Follow up with something like "I got drenched when I went out jogging, but I still went because I enjoy it - do you do any sports?" This can lead to a conversation about sports and fitness.
"I like your dress/bag/necklace/whatever, the colour/style really suits you", or "That's a nice mobile phone, I'd love one of those" (people love compliments on their possessions or looks). This can lead to a conversation about whatever you complimented.
Say something specific about where you are or what's happening. "Did you enjoy the play?" "I thought the last presentation was really good; what do you think?" "I really like this building, it sort of reminds me of the architecture I saw in Italy - have you ever been there?" "This is my favourite song - what sort of music do you like?"
Notice that you're always asking the other person a question that can't be responded to with a yes or no - they have to tell you something that you can then follow up on. Similarly if someone asks you a question, don't just say yes or no - give a bit more detail so it turns into a conversation. If someone doesn't speak to you, don't sit around waiting - speak to them! They'll probably be glad that you started the conversation and they didn't have to. Half of your problem is that you sit around waiting for people to speak to you, and they never do because they assume you're a snob - you need to take matters into your own hands, however scary that thought might be. Force yourself to think of something to say and speak to someone! If people are already talking, listen and interject with your own comments: "Yes I agree with that..." "Something similar happened to me once..." and so on. Push yourself into the conversation, don't hover around waiting to be spoken to.
Since I've been feeling more confident I've actually been known to approach complete strangers and start a conversation by saying "Hi, my name is Thornton" and then follow up with something like "Isn't it quiet in here? I'm glad there's someone here to chat to!" or "Where's your accent from?" or some such comment. I felt absolutely stupid, but it worked! Even something as simple as approaching a group of people and saying "Do you mind if I join you? I'm on my own/waiting for a friend" works wonders.
I guess the main thing for me was getting up the nerve to actually speak to people and thinking of something to say - having a couple of ideas up my sleeve helped a lot, but unfortunately there's no way to become more confident other than by pushing yourself to speak to people and observing the positive consequences, which then makes you more confident to do it again.I hope this helped a bit
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