Thanks for the suggestions, guys! I've already been googling and found some useful links:
Exercise for Beginners
Joining a Gym? How to Navigate the Gym
Also watched some instruction videso on youtube.
In fact I stumbled upon japanforum during that search
But I guess additionally I'll need to find somebody in the gym who can tell me if I'm doing something wrong during stretches etc. (feedback - that's the crucial thing one would miss when learning and trying out himself/herself. )
Stretching Exercises - Total Body Stretch - Exercise
or to help with the group classes
How to Navigate the Gym - Group Fitness Classes
gambarimasu!