Just curious. what sorta riding experience have you had?
first thing I would recommend you do is get your license sorted, if you want to keep it legal, your vehicle has to be registered to ride it in a state forest (permitted areas). if you don't want to go down the path of license and registration, you're limited to private / club property.
If you've got your open car license you can apply for RE-L (moto learners) once you've got that, you can do a 1-day q-ride course ($350ish) if you've had motorcycle experience, or a 2 day if you haven't (not sure how much 2 days costs). And on completion you'll be able to get your RE-O which is your restricted opens, which basically means you can ride unescorted on restricted or LAMS approved motorcycles, which wont be a problem because pretty much every single trail bike you can imagine falls under LAMS.
As for riding spots, there are some places Ipswich way, one near beaudesert, just past the bridge (google it), apparently its legal and free there, even for unregistered bikes, but it is mostly sand from what I hear and that can be a bit tricky if you haven't ridden on it before.
I go to glasshouse alot and that place is awesome for bikes / 4x4s, its about 1.5 hours for me to get there, but I reckon its worth it. You'll need to be registered if you don't want any trouble though, I see police up there all the time getting people for riding non-registered bikes, usually at the entrances. And there is also Mt. Mee, out past Dayboro, you'll need a license there too.
Places closer to the GC I'm not really sure of to many, but would love to find out more as its closer to me, mount tambo has some tracks, I'm not sure exactly where they are but there is a club that does monthly trail rides, rtbc.net.
trail riding is fun as f***