It used to be the case that when you were looking to trade forex, you needed large amounts of money. It's still true that a single currency pair is defined in terms of lots and each "lot" costs a six-figure sum. If you wanted to trade a specific currency pair, you technically needed to purchase one of these lots. Of course, unless you were very rich, you weren't able to do that on your own.
Fortunately, forex sites allow you to use something known as leverage so that you can actually make a trade without needing to invest a six-figure sum at all. In simple terms, leverage allows you to use something small to control something much larger. So, in our example, forex leverage gives you the ability to take a small amount of money and control a much a larger amount in the currency market. Effectively, leverage gives you the opportunity to still bet on the forex market without already being rich.
For example, if a broker was offering you leverage of 100:1, this would mean your starting stake has the ability to control something 100x larger than its actual value - i.e. your AU$100 would be capable of controlling AU$10,000 in currency. Each FX trading site will have its own rules on leverage, but the most common amounts you'll find are 50:1, 100:1, 200:1 and 400:1.
So, if we look at 50:1 as an example, it would basically have this effect on your account:
For every AU$1 you have in your account, 50:1 leverage would allow you to place a trade worth AU$50.Therefore, if you deposited AU$300 into your account, you would have the ability to trade amounts up to AU$15,000. Of course, that doesn't mean you have to use your full bankroll on a trade worth AU$15,000, it just means you theoretically have AU$15,000 to use in the FX market.
Leverage, but don't go too high...
Now, although leverage is great for allowing you to trade large amounts with a small bankroll, it can have its risks. Because you're essentially magnifying the size of your investment, small shifts can have a dramatic affect on your profit/loss. Essentially, the higher you set your leverage, the more risk you assume because any pip movements will be multiplied by a larger figure.
The trick here is to find an amount of leverage that allows you to trade at a meaningful level (and make meaningful returns), but not so high that a single movement could wipe out your bankroll.