background information: Im 16, not super tall, avg looking kid type person. dude was tall at least my age, 6ft. I live in Australia, this happened in the city

Scenario: I was leaning against a wall outside hungry jacks using wifi, look up and make eye contact with some dude walking past me, must of locked too hard or something because he gives me a dirty look, then i look back at my phone cos i didnt expect him to want to start something. comes to say "got a problem?" in a threatening tone I reply "nah man no problem" i looked him in the eye and I think i held my frame solid. he walked away and i looked at him as he walked away?

How do I deal with people like this? In Urban Australia its really common for this sort of thing to happen and I wear expensive clothes and people here are known to stand over you to take what you own. how do i make sure people dont try to take my stuff, or fight me for making eye contact?