Hello Tanha, can't give you expert advice, but I had to chime in just in case you are about to make a mistake.
First, as far as I know, you do not (at least in US) give your paypal account number, if this even exists. You give your buyer an email address which is registered in your Paypal account. The buyer can then send money to this email at Paypal, and you will receive it.
You say you already have an account. Log in and check that the email paypal has for you is correct. This will be the email paypal will use to notify you that a payment has been received. This normally happens very fast if the buyer used a credit card, or sent you funds from their own paypal account. It may take a few days if the buyer used an e-check. Either way, paypal will send you an email, so double check that the email in your account is correct.
As far as linking your paypal account to a card, I can't say. In the US, you enter your bank information in your paypal account. No transfer takes place between your paypal balance and your bank account, unless you have made a paypal purchase and there are not enough funds in your paypal account to cover it.
Apart from this, I believe that all transfers between your paypal account and bank have to be done manually. From paypal, you can tranfer funds to/from your registered bank account.
Also, as just some personal advice, paypal is a big a complex system. Sometimes unexpected things can result from buyer/seller/paypal interactions. It might be wise to have a separate bank account just for dealing with paypal business, and not expose your main bank account with all you money in it. Just in case :).
