Safari

Also check System Preferences/Users & Groups/your account to make sure it isn't in your login items.

You may  have adware/malware installed. Try downloading this program which was written by Thomas R, a long time poster. The program will search for malware/adware. The program will do the work for you which makes it easy. It may be necessary to run more than one scan. After running the scans, restart the computer.

Malwarebytes Anti-Malware for Mac      10.10 and later

Malwarebytes uninstall

Right click (or ctrl click) on Safari's Dock icon and uncheck -> Options -> Open at Login