I think I may have a virus or something, because when I installed a program a few days ago it said something about "www.searchpage.com".
How can I fix this?
Yes, Malwarebytes just came out with a new update. Download this and have it scan your system. It is best malware scanner and works well for me. It said I had no malware and indications were I didn't. But reviews claim it works well.
Also, look in the system preferences for a "Profiles" section. You will want to delete any and all profiles you don't understand why they are installed and what they do for you. Many businesses and MDM install a profile so they can remote wipe and enforce settings. Apple makes sure they explain what they do in that app when you have one installed so you can decide to remove them or question the necessity of some of the choices if you so care to.
Go into system prefs>profiles and see if there's one causing the problem. Uninstall weird extensions in Safari>Preferences. Finally, try a Safe Boot (https://support.apple.com/en-us/HT201262) and restart.