How do I clear past searches on yahoo on my p.c?

There is an option in your browser to do exactly that. Click on the menu icon and find it. Also notice what other options you have. When you are done with that, do the same for your operating system and every other program you use. For instance there is an option that turns your monitor picture sideways, in case you might want to do that. This is called "learning how to operate a computer."