happyvv,
there are many centers ga! e.g. mei foo, TST (彩星,喜來登), tuen mun, shatin, tsuen wan, kornhill, causeway bay (利舞台,伊利沙伯大廈). i usually go to mei foo, the center is very small, but it is close to my house.
if u r not sure u want to join it or not, u can try 7日試玩 (not include aerobic class). have u got any friends who joined Physical? u can ask him/ her to bring u there. u can try 7日試玩 and see whether it is suitable or not~