how do i make google my homepage on safari not yahoo

An article Set your homepage - Apple Support

Set search engine as google by typing the google link in home page shown with red arrow and then follow the steps in the article .

Setting Google as Homepage in Safari

Through Safari browser you get to the menu bar and select Edit for editing the preferred selection. Here you will find Preference tab from the drop-down box that will take you to the general setting where you can enter www.google.com at the empty space where you can see homepage. By the time you have selected your choice the system will ask for allowing it to apply the same.

Source: waftr - Make Google my Homepage

Try going to Safari Preferences --> General

If this doesn't work, you might have malware/adware on your computer that is forcing your homepage to be Yahoo rather than Google. In that case, you have two options to fix it:

• Download Malwarebytes' Anti-Malware for Mac. It was developed by a trusted and respected contributor here. It's a simple, non- intrusive program that deletes known malware/adware from your hard drive. That's all it does. It doesn't add anything and it doesn't take away anything else.
• Follow the directions in this Apple support article to remove it manually --> http://support.apple.com/en-us/HT203987

Go to Safari Preferences/Extensions and turn all extensions off. Test. If okay, turn the extensions on one by one until you figure out what extension is causing the problem.