I really don't know how to sign a message. What is the goal of signing a message. Thanks !
To prove that you indeed have ownership for specific address. On bitcointalk, people use signs to verify users' ownership.
Basically this. E.g. in the first post I sign a message with an address I posted when I was still a noob, thus it is reasonable to assume that I am still the same person posting. This can be important later if you want to take a loan with your account as collateral, want to sell your account or if someone suspects you have been hacked.
It can also be useful to sign the terms of a trade. Thus neither you nor your Partner can later claim they did not read a specific detail oft the trade.
It is important to include the date, thus the message can not be reused at a later date by someone else. Some people also include a random phrase.
As almost everything this is not 100%, because I could have sold the shorena account, this alt account and all addresses and corresponding private keys associated with the accounts.