You can trade with $5, depending on your leverage. 

For example, if you have 1:500 leverage you could buy 0.01 lots of GBPUSD at 1.23322 and the trade would cost $2.47 not including commission fees. Whereas if you did the same with leverage of 1:100 it would cost $12.33 (again no commission included).

You would be limited in the instruments that you could trade as some have higher contract sizes or lower leverage. Indices and commodities have lower leverage so you would require more money to open a position.