There are a number of signs that may indicate that your computer is compromised. You start seeing lots of popup ads. Your default search engine has been changed to something that you do not recognize. When you try to go places on the web you are getting redirect to other sites.
These are signs of adware. Adware is gray software meaning it is not out to directly extort money from you directly, its purpose is to redirect where you spend your time and money on the Internet. Adware is usually bad written and often can slow your device to a crawl. Adware sometimes will be used to load nastier software like ransomware where your files are hidden from you until you pay a ransom and then you may regain access to your files.
It is import to remove this software if you have it on your computer. You can search for malware scanners for your operating system on google. For windows 10 there is windows defender which scans for malware, for older versions of windows you can download Microsoft Security Essentials for free, and for the Mac you can get MalwareBytes.
It is important to keep your computer free from adware. Adware will cause your computer to behave unexpectedly and may serve as a gateway to more malicious software. If you cannot remove the software yourself you can reach out to local computer repair specialists.