As shallow as it may sound, the first thing that you need to think about is what you're going to wear. There's a lot of truth to the phrase "when you look good, you feel good." This doesn't mean that you should necessarily go out and buy a new outfit. By all means, make sure you wear something that is genuinely "you" and that looks good, but don't try to be something you're not. If you're unsure of what you're wearing, it'll make you feel all the more uncomfortable. However, if you're feeling a bit adventurous, a quirky article of clothing can always serve as a good conversation starter.
Mindset is crucial--so many people get caught up in thinking that everyone else is paying attention to them and judging them that it cripples their ability to interact. Contrary to this commonly held belief, it is more likely that no on is paying attention to you, unless you are actively talking to them. Truth be told, just about everyone in any given social setting is so busy worrying about themselves that there's very little attention left for much else. Take comfort and ease in this fact, because it's true.
Most parties serve some kind of alcoholic beverage, so if you know how to handle yourself under its influence, go ahead and have a drink or two. Alcohol can definitely loosen you up enough to feel sufficiently comfortable in a social setting. Beware, though! If you drink too much, you can end up making a fool of yourself. You don't want to be remembered as "that guy who knocked over the punch bowl climbing onto the table to dance and then threw up on everyone's heads."
If you're nervous about ending up in a corner with no one to talk to, bring a friend or go with a group. When you're surrounded by at least one other person that you're comfortable around, you'll have a much easier time acting like yourself. Mingling in groups can make meeting new people much easier because there's usually one person who's more extrovert who will then introduce a new face to all their friends.
If you end up having to fly solo, there are a few safe bets for who to target with conversation. At every party, there's the wallflower. This is the person hanging out by himself in the darkest corner, feeling a bit shy and nervous. While this person may seem standoffish at first, he is actually dying for someone to come over and talk to him, and probably has more more worthwhile things to say than most. The social butterfly is also a safe bet. This isn't the same as the token "hot" guy or girl that everyone seems interested in. This is the person who is a natural-born networker and sincerely nice person. You don't even need to initiate anything. Just stand in the general vicinity and she'll get to you eventually.
Say you're the type who stumbles all over your words in normal settings, and this party thing really exacerbates the problem. In that case, it might not be a bad idea to make a few mental notes about some upcoming events you're interested in, or perhaps something that's going on in the community that you're involved in. This will give you something your mind can automatically jump to at the onset of an awkward silence. You can also prepare some really generic questions for when your mind goes blank, such as "What are you studying?" "Where are you from?" or "What kind of music do you listen to?"