If I become member of a fitness center online and I accepted the general terms and conditions, but they did not mention house rules, am I legally accountable if I violate them and maybe more import, am I allowed to quit the contract if otherwise I would be bound to it for 1 year?
They are not required to verbally notify you of this aspect of the contract, but it has to be part of the contract. Suppose the contract say that you may use the facility during open house for 1 year, and you must pay $1000 for the privilege: it says nothing more. They cannot then unilaterally rewrite the contract. This is why they almost certainly have additional provisions in the contract, which could allow them to modify the contract in certain ways, or specifically they may have a requirement to "comply with house rules". You can look at the written contract and figure out exactly how house rules become enforceable. You can quit only subject to whatever the terms are that govern quitting. I do not expect that it is a trivial matter to locate the terms that you agreed to, or any separate "house rules" linked in that web page.