in safari when I click on something, it opens up an ad in new tab

I have an iMac, 2013 model. operating on OS X 10.9.4 and whenever I click on just about everything in safari it opens a new tab with an ad on it.

sometimes the ads are mackeeper or video hosting sites but they are just so annoying! I'm wondering weather I downloaded a virus or something...

how do I get rid of viruses? there seems to be something wrong with my Youtube too, a blank space keeps coming up under the video and every 2 or so seconds it gets bigger so that the comments and video info just keep going down! it also happens on the side where the video suggestions should be!

please help, I'm so frustrated! thanks 🙂