Congratulation!
The first one is always hard, you don't really know what to expect...but I'm sure you will do fine.
My approach to these types of social gathering, is to be a better listener than a promoter of your business (of course market yourself though!). You just want to keep in mind that everyone will be there to talk about their business... and this is where I use my listening and evaluating skills to hone in on the specific things.
You want to get people talking about what they do and how they do it so that you can be able to suggest things that you can help them out with...It's not just about telling EVERYBODY what you do in your new business, but I think it's better to be able to tell a SPECIFIC person, what it is that you can do for THEM for their business.
One last thing is that when you collect business cards at the function...dedicate a good hour the next day to sending everyone a quick "nice to meet you" email...it sends a great impression to someone that may need your service one day or if they know someone who could use it, they are more likely to forward the email to their colleagues! Be sure to add a link to a special discount of some sorts in your email.
Thumbs up to you as you go out!