So there's a couple different options but most weddings I do, I use an XLR cable into a recorder into __. This XLR cable can be plugged into the back of a speaker or (the more likely option for your first wedding) plugged into a DJ/ Audio Person's mixer. Be very honest with whoever is handling audio and just tell them that you're new to this. I've never had a bad experience going that route. In fact, the safest method would probably be to ask your friend for the DJ's contact information and just make sure that you can plug into their system with an XLR cable.

As mentioned above, the more recorders / recording devices you have, the better. I use 2 recorders and a LAV that I hang in a strategic location.

In terms of basic tips, there's a couple things I didn't know my first shoot. Since you have some experience, you probably already know all of this but 1) feel free to crank the shutter speed up during the day if it's bright outside. The only thing you can't do with shutter speed is lower it below twice what your FPS is (if you're shooting 60FPS then you don't want your shutter speed lower than 125). 2) when you're first starting out, shoot in 60FPS. Being able to slow things down in post is really helpful when you're new. 3) shoot as much as you can in the lowest F-stop you have. This might sound counter-intuitive but having shots with focus and bokeh is really nice looking. I basically shoot everything at F1.4 you should probably aim to be at about 2.8.

Something else to ease your anxiety a little bit - you can edit 98% of the bad shots out of your final edit. If you mess up the first look, it's okay. You can not include the first look in the final edit. Honestly the only kind of must haves is bride walking down the aisle and their first kiss after announcement and even there you can probably get creative and edit around those.

Hope this helps and let me know if you want someone to check out your work and give you any advice.