How to make a great first impression at a conference?
Don't try too hard. You will have plenty of time later to build a solid reputation.

First impressions are way overrated. They will not be the basis of any serious (as in "costs real money") decisions, least of all concerning some complete newbie.

Enjoy the conference, get to know people, mingle. Play your rôle of newcomer who is just joining the club. Check out what is going on in the sidetracks, you might be quite wrong about their importance (or totally right).