it really depends on the coach and what kind of rotation you are doing, if you find out wha type of rotation you are you (ex. 6-2 5-1, ect,) then i can help you.
but the general idea is you rotate around the court (which im guessing you already know) and when the ball is on the other side of the et, you are up by the net, ready to block, in yur case you would be i the middle. you still rotate around the court the normal way, and before the ball is served, you must be standing in the spot yo would be in acording to your rotation, but after the ball is served, you run to your spot that you play (in your case the middle) and play that positin.
when the ball is on te other side of the net, you must be ready to block, usually the middle will block everything. when a free ball is coming over the net, you will transition back to the 10 ft. line, and be ready to play defense. you also do the same when the other team is serving.
okay so i really hope you understand this, i am very bad at explaining things, and im not even sure if i understand it myself. haha.
hopefully this is not to confusing.
but it might take a while to learn, but just ask your coach or another player that knows the rotation well and they can explain it to you, again it really depends on what rotation you are doing.
i hope this helps!
good luck! (;