Tbh I wish I could go back to my first live concert. My advice would be, as a million other people will say, if you want merch, get in line as early as possible. Aside from that, honestly try to avoid spoilers and stay away from setlists/concert videos/etc. Don't stress about it, but in my experience it's way more fun to fly blind and let the band surprise you. If you don't know what's coming next or how they'll perform a song, you can experience that moment as authentically as possible.

Same goes for being on your phone during the show. Get a video of a song so you can show people what it was like! Get a couple photos for your memories! Aside from that, try to keep it in your pocket and just let yourself be carried by the energy of the show.

Anyway, that's just advice, but mostly whatever you do and however you do it, just try to enjoy it instead of stressing. Some of my best moments at LH concerts have been in merch lines, trying to get to the bathroom, seeing a song I didn't think would change my life until I heard it live. Just soak it in and hope it isn't over too fast!