Being nervous and tense is normal. I get it all the time before a race, even after having started in many races, often even before shorter just-for-fun races. When the starting signal sounds, the nervousness immediately falls away, and your head will get into it, don't worry. :)

I've not ever done one, but I guess remember that you're doing it for yourself. Just completing it will be an achievement in itself. Good luck and remember to have fun.

I've run three halfs in the last year. Biggest thing for me is focusing on my stride in the second half. As your legs start to weaken, your stride will suffer which may leave you at risk of an awkward step that could have you dealing with rehab in the weeks after the run. For me personally, a single odd step (imagine going down stairs and thinking there's one more at the bottom when there's not) has sidelined my training for almost two months now. I can still run, but I can't push distance at all. It's to the point where I'm considering another couple weeks off even after a month with no running after the half.

Keep your eyes on the upcoming terrain and make sure it doesn't surprise you.