Please Help! I don't know how to get rid of this extension!!

I purchased this mac book pro in the middle of 2014. So it's not even a year old. About 4 months after purchasing I noticed that I started to get pop ups that were opening new tabs. Particularly 'Mackeeper'. I don't know if this is an extension or what, but I have been looking for an answer but can't seem to find one. Every 3rd or 4th click and BOOM another pop-up of either an 'airpurifyer' add or a video game add. It is driving me round the bend! I've now noticed that on webpages, words have started to be hyperlinked. Please help before whatever this is destroys my macbook.