How can I see what applications start when booting up my Mac?

It may vary. Some you can see in Preferences > Users > Login items for a specific user. Others depend upon what was open when you shut down and at that time told the computer to reopen all windows in use at shutdown time.

loiterer330 wrote:

How can I see what applications start when booting up my Mac?

Command Tab will present all the running applications,

this will include all the the apps you have choosen for Log-in items

>System Preferences>Users & Groups>Log-in items

unless it is something like a 'Dropbox,' which registers in the menubar. I am sure there are others...