Safari
How do you stop Safari from automatically popping up when you start up my computer (MacBook Pro)?
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
