Microsoft Word and Excel open on start up
How do I prevent my Word and Excel opening when I power up my I mac?
iMac
If this is on a recent Mac and OS, the default setting automatically opens windows and apps, that are open when you shut down, when you start up next time into the same user account. This is the Resume feature. Therefore, if Word and Excel are open when you shut down, they will open when you start up.
If that's not the cause, check System Preferences Users & Groups pane Login Items tab. If Word and Excel are listed there, remove them. These are things that run automatically at log in into your user account. The only item that's there by default is iTunesHelper (don't remove that one); if you see other items there you no longer use, remove them.
This document has additional details
Automatically re-open windows, apps, and documents on your Mac - Apple Support