1.install putty-tools with the following command:
    sudo apt-get install putty-tools
2.Now convert your .ppk file to .pem using the following command:
    puttygen yourkey.ppk -O private-openssh -o yourkey.pem
3.Set the proper permission to use the .pem file with following command:
    chmod 400 yourkey.pem
4.Connect to you server using the following command
    ssh -i yourkey.pem serverusername@server-ip