You can trade with $50, 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 $2.57 (not including commission fees). If you made the same order with 1:100 leverage, it would cost $12.33 (not including commission fees).

You may be limited in the instruments you can trade as some have higher contract sizes or lower leverages. Indices and commodities have lower leverage therefore, more funds are required to open a position.