Congrats on the great swimming! I wouldn't worry too much about nationals in your first year -- I considered myself lucky to make that meet by my senior year of high school!
On the nerves front, I think developing a routine is pretty important. Whether that is walking up listening to music, doing the crazy slap thing, doing the same stretches behind the block, routinizing the swim and making yourself think it's like any other race might be helpful. Watching Torri Huske swim when I swam with her, she did the same thing every time, she was in her zone, doing her things. I definitely got nervous before big swims (especially when my time to get a nats cut was running out) but one of the things I loved about swimming was you pretty much just hop in the water and push yourself as hard as possible (with some race strategy). Not too much to stress over -- you should hop in, black out, swim hard, and see what the result is.
Good luck!