1. Go to Settings at the top right corner of your Gmail page
3. Find Signature tweaks feature and enable it by selecting the radio button against Enable button
Note: When this Signature tweaks Gmail Labs feature is enabled, it also places your Signature at the top of all the chain of messages to which either you are replying to or forwarding. By default Gmail places the Signature at the bottom of all the set of messages when you are going to reply with history or forward them.