how do I get yahoo off my macbook?

OK, that sounds like it may be adware.

I'd suggest, running Malwarebytesand see what it turns up.Malwarebytes was developed by one of our own colleagues here in ASC and is about the most proven anti-malware software for Mac.

If it is the search engine you mean then go to Safari menu > Preferences > Search and change search engine to Google.

If it is the browser itself, go to System Preferences > General and change the Default Web Browser to Google.

Which web browser are you using?