While the message is the most important part of any meeting, it's important to know the protocol too. Excuse me if this seems obvious, but I've been in more than a few meetings where some clueless dork ended up looking like they just stepped in dog doo on the way in.
Before the meeting: Ask your boss what's the dress code for the meeting and if you should bring any particular materials (e.g., special handouts, etc.)
At the beginning of the meeting: Before you enter the room, inquire of your boss where you should sit...probably next to your boss, but he/she might suggest another seat, e.g. across from your counterpart of the other company. If you are not arriving with the rest of your group, be the first one at the site. The most senior person should lead the way into the meeting room; as the most junior staffer, you should be the last one in and close the door behind you. If you are there before the other side arrives, you should stand up when they enter. Let the VPs lead the introductions. If the group is small, shake hands with the members of the other side and exchange business cards. (unless you can remember all their names, either refer to their business cards or list their names in your notes so you can address them by name if called upon.) If coffee/refreshments are available in the meeting room, hang back until the others have served themselves before serving yourself to a bottle of water or cup of coffee only....and take a napkin to put under the cup/bottle. Take your cues from the others about removing suit jackets, loosening ties, rolling up sleeves.
During the meeting: As others have said, you're there because you have information to share. Answer any questions directed to you briefly and pleasantly. If you are making a Powerpoint presentation, keep the number of words on each slide to a minimum and NEVER turn your back to the meeting to read from a slide. If you're sitting around a table for an informal presentation and you have handouts, either pass them to the right and left or walk around the table handing them to each person. Do not start your presentation until everyone has their copy, and make sure the pages are numbered so you can direct them to the right spot. If someone else is making a formal presentation, as the junior staffer you should manage the lights if they need to be dimmed.
After the meeting: Stand up and shake hands with the other side, holding back until your senior person makes the first move. Assuming you will have a debriefing with your boss or the entire team, make sure you know your next steps. You might be asked to summarize the meeting in an email or as meeting notes; if so, do them as soon as you can. Thank your boss and his/her boss for including you in the meeting.