what's the principle of sending money and receiving money? who do the work to put money into account？ If i want to send money, Who gives the payment address. You know, it is said there are no center. 
Another question is that if someone send money to me, who help me  put the money into my account?
An individual or merchant requesting payment will generally want to obtain a bitcoin address specific for that transaction.  When done manually, this is obtained from the Bitcoin-Qt client by clicking "Receive".  A merchant would have software that would query the Bitcoin.org client to obtain the address. An EWallet provider will provide for you this address.

The concept you possibly are misunderstanding is like how with a bank account, the account is assigned to an individual or organization before payment is transferred.

With Bitcoin, the client uses math to create the private key (and from that determine the Bitcoin address).  So there doesn't need to be some master registry where Bitcoin addresses are stored.  The person's client simply holds the keys necessary to spend any funds received by an address.

So that answers your second question -- bitcoin never really exist "in your account" but instead if you have the private key for an address, you can spend any funds received by that address.  Every node performs verification of your transaction and won't accept a transaction in which you try to spend funds that aren't yours.   Nodes that are mining will make these transactions permanent by including them in a sequence of data blocks -- which is named the blockchain.

So there are no people doing work, and instead there is simply an electronic communications protocol that is being employed.