I need to install GNUPG on Ubuntu Linux.I followed the step but am confused where is the private key while it only shows the public one.
Is Secret key a private key?If so why are they same?

Have you read the GNUPG Doc

The Public key is used by the client you are communicating with so they can encrypt/decrypt the message.
The Private key is kept secret as only you should have access to this key.

What shows only the private key? Did you try:

gpg --list-secret-key keyid

Yes, they are the same. Why shouldn't they be?

Public key crypto is based on key pairs. You have a secret key which is used for signing and decryption. You keep this key secret, on your computer. The other key in the pair is a public key, used by others for encrypting to you and verifying your signatures. You can mail this key to people, link it from a web page, or put it on public keyservers.

You need to do some research and understand this tool or you will do something bad to yourself

The proper doc links are at http://gnupg.org/documentation/index.en.html

The doc referenced in the post above is not up-to-date. Maximum pubkey sizes in current versions are 4K.