Your first bike could be anything from a 50cc scooter to a 650cc V-twin - the choice is yours.
So you want to buy a bike? Your first bike in fact, and you are a little confused. Well, that's understandable. There are so many different makes and models, not to mention completely different 'types' of bikes. A scooter for example, may not technically be a 'bike', but it may be a better option for you than a conventional bike.
You may say, "I just want a road bike" but even road bikes came in a wide variety of 'flavours' such as commuters, sports bikes, or cruisers. Dual purpose road/trail bikes are another option than can make a lot of sense as a first bike but, like everything above, they won't be for everyone.
Assuming that, if you are looking for a first bike, you are either a learner or a novice/provisional rider, you can't just jump on any bike and ride off into the sunset. All the Australian states (bar Western Australia which has a 250cc restriction) have in place what is called the Learner Approved Motorcycle Scheme, or LAMS for short.
Initiated in NSW back in 2002 and gradually adopted by the other states since that time, LAMS restricts learner and novice riders to bikes under 660cc and with a power-to-weight ratio less than 150kW/tonne. If that sounds complicated, don't worry because your state government road transport authority (the RTA in NSW, VicRoads in Victoria) has a list of LAMS approved motorcycles (both new and secondhand) available on their respective websites or via their offices. Essentially, all bikes or scooters of less than 250cc are okay (with the exception of a handful of high performance models) while those between 250 and 660cc are approved or rejected depending on their power output and weight. If it's got a high performance four cylinder engine, than you can bet it's out but if it has a lazy old single or twin cylinder engine, then it's probably in. But you need to check.
Before this scheme came into play, the general learner/provisional cut-off point was 250cc, but this caused a number of problems. Some 250cc bikes, namely the high performance two-stroke twins like the Suzuki RG250, Yamaha TZR250 and similar models from Kawasaki, Honda and Aprilia were anything but learner friendly. At the same time, bigger or heavier riders, or those who needed to travel longer distances, weren't necessarily comfortable on smaller bikes so they needed to buy one bike for their 'L' and 'P' licence and then another once they obtained their full licence. Under the new scheme, something like a 650cc twin can do both jobs and provide safe and satisfactory motorcycling for many years.