How will I remove the virus on computer?

How will I remove the virus on computer?
I was using my computer then suddenly a virus popped up on my screen. It says that my computer has detected a virus and it's a harmful one. I'm trying to close it but it always comes up. I restarted my computer to see if it's going to remove and then after I restarted it, I waited for about 15 minutes before I started what I was going to do. After that it comes out again.
My Antivirus is Avira and it strongly detects the viruses but it depends on the virus and on how harmful it is before it can be cleaned or deleted from my computer.
What should I do now? How will I remove the virus?
