Why, then, do you think so many professionals regularly fail to achieve it?
In trading, I would think something like $500 (maybe $1,000, if you can afford it), is a good starting-point.
As helpfully commented on above, if you go somewhere you like Oanda, you can get infinite flexibility of position-sizing, so the amount itself doesn't really matter much: you can scale down any position-sizing method to any size you want, to get started.
But that's trading - not "learning to trade", which you can - and perhaps should - do on demo accounts without risking a penny.
I used demo accounts for 4 years, before opening a real one (I was under-age, when I first started learning forex-trading), and that stood me in very good stead. The education-process comes before the actual trading.