Depends on exactly what interaction you are speaking of - is it at a networking event at an interview, or something else?
The key rule is a first impression won't get you the job, but it could set you back significantly if you show poor discretion. So just be calm, composed, and professional. Use typical professional mannerisms such as a solid handshake, look the person in the eye, smile, etc. Don't try to be too funny or become best pals with your the interviewer or person from the firm you are interested in. Avoid confrontational or sensitive topics (such as political, social issues, etc) , but do ask questions related to the industry of the person you are speaking to or questions about the firm which show you have done your homework and taken the effort to learn a bit about them beforehand.- thoughtful questions go a long way. If it's a networking event, obviously don't drink too much or anything else you wouldn't do at any other professional event.
You don't need to hit it out of the park in your first meeting - no one ever does and it's not important. Don't be the life of the event if that's not just you - those folks may seem impressive but end of the day, that's not important in getting an interview or the job. Be professional, curteous, and don't do or say anything stupid. That's the baseline. Beyond that, do your homework beforehand and have some insightful topics or questions to talk about that show you have taken the time to understand the firm you are speaking with. And finally - just relax! :) It's pretty hard to make a bad first impression, so I'm sure you will be fine.