I was able to be an alter server when I was in high school (when they finally let girls do it). I remember being kind of nervous the first few times I served, but then it didn’t bother me anymore.
As for the scheduling: at my church there was a person who was responsible to schedule the servers about 3 months out. We would let her know when we were going to be unavailable and she would schedule somebody else for that mass. At my church there were 3 alter servers per weekend mass. The week day masses were done by 1 or none.
I would definitly talk to your parents about it. I enjoyed doing it and I think that it makes the mass that much more meaningful.
Good luck!!