If an agent likes your script enough to meet with you, you've already won half the battle! Agents will not bother with a writer unless they think that writer is talented. Think about it...it takes a lot of work to get a writer a job (or sell their spec material). Agents don't get paid until you do, so signing someone is always a risk and an investment of time.
In the meeting, all you really need to do is be passionate about your ideas. (So I guess that's the first thing...have some ideas.) When it comes to an agents and managers, you want someone who is excited about you and excited about the kinds of material you're excited about. Someone who gets you as a writer.
As a general note, I would do as much research as I could on IMDB Pro, StudioSystem, Variety, etc., about the agent's background, clients, etc. This is actually good advice for all meetings. You shouldn't feel like you need to impress them or kiss ass (they're already impressed by your script), but you don't want to come in and look too green or clueless about how Hollywood works. Agents want to represent people who are ready to be sent out on other meetings.
