Step 1: Generating SSH Key
1.From the command line, enter ssh-keygen. (if you get an error in the command window, check if C:\Program Files\Git\usr\bin is part of your path) The command prompts you for a file to save the key in
Press enter to accept the default key and path, /c/Users//.ssh/id_rsa.
Enter and re-enter a passphrase when prompted.
The command creates your default identity with its public and private keys.
Load the ssh key generated previously(id_rsa). it will prompt you to select the file. (default path of the ssh keys C:\Users\.ssh)
While loading it will ask for passphrase.. enter the passphrase that was given earlier. After this step putty will load the certificate.
Once the certificate is loaded, click on the 'Save private key' button. it will prompt for picking the folder in which the file to be saved. Better to keep it in the same one as ssh key folder (C:\Users\.ssh). specify the filename also.
Now you have to configure the TortoiseGit with the generated key file. right click on the repo folder select TortoiseGit>Settings
In the settings screen select Git > Remote
In remote screen, select origin.. it will populate the URL and other fields.
Click on the ellipses against the Putty Key field and select the private key file that was generated with puttygen.
The file format of the private key is
----BEGIN RSA PRIVATE KEY------
The error was
Couldn't load private key(unable to open file)