You are able to trade even if you had $5 in your account. It all depends on your leverage.

If your leverage is 1:500 you could buy 0.01 lots of AUDUSD at 1.12437 and the trade would cost $2.25. Whereas if you did the same with a leverage of 1:100 it would cost $11.24.

You would be limited in the instruments that you are able to trade since some have higher contract sizes or lower leverage. 

Indices and commodities have a lower leverage so you would require more money to open a position.