How do offers work?

Do you need to accept your first offer? Can you see if you get accepted into other courses?

If I get into VU Early entry and have now met the prerequisites (20 in English), does that mean I’m definitely in? I didn’t set it to first preference as I was told by my careers teacher it didn’t matter. Does that mean if I get accepted into preferences before it, I can’t accept my VU Early entry?

It’s not the end of the world, but I mainly set my first preferences to see if I could get into that Uni and then decide, but realising basically if you get into your preference, then that’s it…?