With regards to how much you need to start trading... I would say that how much do you have to lose is a good place to start. But putting that to one side we need to look at the practicalities.
Some brokers let you deposit $5 to start (although not many) but depositing $5, $10, $20 or even $50 is probably a bad idea because it is very much related to what the minimum trade is... which with these types of accounts is usually 0.01 lot or $1000 before any leverage is applied. Therefore you cannot properly trade with small amounts. On top of that we must also consider that if your balance is capped you will get stopped out very easily. In other words the smaller balance you have the more likely you are of losing the whole lot.
As a guess I would say most experienced traders play with no less than $5,000 balances and often much more, but remember they should be able to afford to lose the lot.