I remember when i first started getting into fishing as well mate,
If you're land based in sydney harbour your best bet at gettin a nice fish would be off any ferry wharf in the harbour, but watch out for when the ferries come in, they dont wait for you to get your lines in!!
Some of the nice wharves are in Balmain, Drummoyne, Gladesville and up towards Watson's bay. It depends where abouts you are based but find a wharf or structure and you're in with a great chance
Cheers,
JB