Safari

Every time I search something on safari or google chrome it goes immediately to yahoo and yahoo is not my main search engine so I don't know how to fix this. It is very frustrating when I need to do my school assignments using google. I was reading online about it and it said something about a melware virus on my computer and I don't even know when that app downloaded on my computer and I deleted it off my MacBook air today so I could figure out how to fix this yahoo problem but it didn't do much to help.