When I want to send bitcoin from my wallet to another bitcoin address in blockchain.info, I must to have a private key. How I can obtain a private key?
When completing a transaction via bitcoin, you never need the target's private key. Never give your private key to anyone. NEVER. The private key is the only secret you have that protects your bitcoin. If you give your private key to someone else, they can "spend" your bitcoin.
When sending money to someone, you simply need an "address"; the recipient should provide you with that address. If you are attempting to send money from one BlockChain.info wallet to another waller on BlockChain.info, you can get the target address by clicking on the "Request" link inside your wallet.
See this BlockChain.info support page for details.