HELP WITH COMPUTER VIRUS!!!!?

Ight so last week every 5 minutes this thing called "windows defender" keeps poppin up saying i have a virus or somthing and everytime i closed out it kept popping back up. I dont really have an antivirus software on my computer. Now everytime im on the internet, if i click a link or even a picture my computer will bring me to a completely random website (usualy ). Whatever it is its makin my computer run slow. I dont know if i have, a virus, trojan horse, ect. HELP i need to know what i have and how to get rid of it. Thanks