How to you remove yahoo toolbar and search engine ???

You may have inadvertently installed adware. You do not need to download or install anything to fix it.

For a description of how this may have occurred, how to avoid it in the future, and for Apple's recommended actions read How to install adware. Apple's instructions are linked in the Recovery Procedure near the end of that document. Read and follow them carefully. Pay particular attention to the easily overlooked passages directing you to restart your Mac when required.