how do I check my computer for viruses?

You can search for viruses for as long as the day is long, but you won't find any as they don't exist.

You may have installed some other malware, but I imagine it is just some incompatible software.

Try booting into Safe Mode and see how it runs. It will be sluggish and some things don't work. Try safe mode if your Mac doesn't finish starting up - Apple Support

If you had MacKeeper installed try to uninstall it and reboot your system and tryMalwarebytes Anti-Malware this may resolve your problem. Try this link as well How to eliminate slow Mac problems

There are no viruses that can attack Mac.OS X. So, anti-virus programs are selling a cure for a disease that doesn't exist. And they are renowned for borking Mac systems.

A Mac can pick up malware but not if you just use common sense. You really have to actively invite some malware in for it to happen. That means downloading and knowingly giving your password for it to download. Mac provides all the protection you need with SIP (System Integrity Protection).I About System Integrity Protection on your Mac - Apple Support

If you do happen to download malware you can use MalwareBytes to get rid of it. Malwarebytes was developed by one of our own colleagues here in ASC. It gets rave reviews and is about the most proven anti-malware software for Mac. If you use torrents, don't. Torrents are a good way to pick up malware because it can hitchhike in on a download package and you’ll never know it.

Safari pop-ups are different and easier to deal with. In Safari 9.1 and later just close the window. For earlier versions force quit and hold the shift key while restarting Safari.

https://support.apple.com/en-us/HT203987

Finally, stay away from programs that purport to "clean your Mac". They can do damage to your system. Your Mac needs no cleaning in the first place. All the cleaning that is necessary is taken care of routinely by OS X.