On one hand, you got in the system preferences, in your account, the aliasses of applications which launch on startup.
Begin with those.

But not all auto-loaded applications are listed there.
A bunch of background applications may be installed by other softs like antivirusses, firewals, mailers and such : Applications which have to intercept things, while they are "not in front", not in the active frontmost window.

To see all which load at startup, click on the apple menu, top left, "about this Mac", and in the popup, click "more information". There you may see all startup applications, and all your "normal" applications, too. You'll be astonished, how many there are - can take a while, to collect the information on disk...
A good way to guide spring cleaning.

Also, you got the"activity monitor" program, in the applications / utilities folder, which can show you all programs which are loaded, or which really are running. And you can stop applications, there.
All - that means really all. Including the parts which make the system : kernel_task, WindowServer, loginwindow...
So if there you stop a running proces / program, be sure it's not a vital part of the system.
If there you see unintelligible charabiah applications running, first try to find out, which program it belongs to, and if it's unwanted, disable via that program.
"Killing" a process via the monitor, is not the proper way to to things.

But things like antivirus daemon is not always needed,
or, iCalAlarmSceduler only is there to remind you of alarms you set in iCal, not really needed to have the puter tick, but might perhaps cause Vue to hickup, if alarm pops up and drrings while rendering - I don't know.
Or some fuzzy automatic update routine desperately want link to the internet, even when not connected.
On an old Mac, never "clean installed", I even found orphan routines running : invisible parts of applications trashed long ago, like some Norton :-(
S'cuse me, if I'm too technical...