I have had 1 shot of Moderna so far. My next is in three weeks.
The injection itself is easy and painless.
About 24 hours after the shot, I had a rough night. I had a headache, what felt like a mild fever (I didn’t take my temperature though), and some chills. Also my arm was sore at the injection point. I had no breathing issues. I had trouble falling asleep but eventually did. When I woke up it was like nothing had happened.
The symptoms lasted about 12 hours after they started (except the arm soreness which lasted a couple of days but was no big deal). It sucked overall but I’ve been through way worse. Not looking forward to the second one, though.
I’m not sure what you could do to prepare. The symptoms, if you have them, are uncomfortable but my understanding is that they pose no danger. And some people have no symptoms at all.