Very late to the party on this one, but in my experience it's less about whether or not you're drinking an alcoholic beverage and more about whether or not you'll turn up without the desire to drink an alcoholic beverage. From my experience (and I'm a drinker so I guess take it with a pinch of salt), it's always been find to just approach it as "No thanks, I don't touch the stuff! I'm sticking with --whatever beverage-- but appreciate the offer!". Just show that you appreciate they are offering to get you a drink but that you are turning it down because you don't enjoy it (I imagine sometimes people get "harrassed" because they think you're turning down a drink as to not allow another person to go out of their way to pay for it or something...they think you're just being too nice/polite so try to goad you into a drink. This is what I have observed at least).
As long as you don't have an issue being around those that drink, there should be no problem as long as you do make an effort to get out and socialise whether or not you're actually drinking alcohol yourself. If anything, you might be more of a hot commodity (if you drive) as you could wind up the DD lol (people DEFINITELY appreciate that) and even if you're not, I've never actually met somebody who balked at a person who just wanted a soda.
If you want to ease yourself into it and are worried about just saying it's not for you, when asked you could always just say "I'm taking it easy tonight" or something similar to that. Then when you get to know people better you can just say that you don't actually drink at all but weren't sure how others would react so you didn't come out and say it right away. Again, I don't think you'll have a problem. Most people now a-days won't be that bothered or try to pry (just in case - for all they know you might have a problem with alcoholism or something). If they give you a hard time, they maybe aren't people you'd want to be hanging around with outside of work anyways?