Can somebody please explain to me how to sign message with Xapo wallet?
You can't. I highly recommend you to use a desktop(or mobile) wallet. If you want use an online wallet, use Blockchain.info. Other online wallets are insecure.
But I need the address on my Xapo wallet to sign a message
btw, why is other online wallets beside Blockchain.info are insecure?
Also, the reason online wallets are considered insecure is that you have to share private key with third party (think of you sharing your home key). Only blockchain.info stores private key in your device (they encrypt your private key on your computer and then upload to server, so you still have to trust bc.info to encrypt your key), that's why MZ suggested you use bc.info (or desktop wallet).
But if we stores private key in our device, it means if we lost our private so we lost everything.
That's why Xapo don't want to share the private key, so if the private key lost Xapo will refund our balance. (CMIIW)
Do you tell another person to create a bank account for you and tell him/her to withdraw money when you want? Or do you create a bank account yourself and never share with another person?
Yeah, that's make sense too. Okay so it's more safe to use Blockchain.info than Xapo, then I will move to Blockchain.inco
btw, I used to use Blockchain. But because there is so many faucets offerig more rewards for Xapo users, so I moved to Xapo
Check out this guide if you want 100% offline security for the coins:
https://bitcointalk.org/index.php?topic=1002719.0
I'm not asking about offline wallet, I'm asking about the best online wallet service. Offline wallet is not compatible with my device
What device are you using? Why not use PC. Sure its the most secure way, mobile phones are all surveyed.