Thank you dear @blocktrades for your valuable advice. I know that trade sites like yours use wallets that provided by the coin company.
I have one more question: if I keep all my coins in a computer wallet and turn off the internet, will that cause a loss?
Thanks
No, turning off internet won't cause the coins to be lost, although you can't spend them until you turn it back on. Similarly, you can turn off the computer and your coins will still be safe. In fact the cold wallet I recommend involves keeping your main bitcoin holdings on a computer that never connects to the internet.