Wow 21 bitcoin, that is enough for trading, i'll sign in this link
www.poloniex.com then if I get profit from trading i'll spend the profit to buy a car. And still 21 btc for trade.
Most of the links are fake, personally i don't believe on it. If we give our all amount only behalf of link, and if some days ago we never get the profit, so what we can do only the link, my mostly friend told me that online trading all are risky.
So, i avoid it, and better is that invest it. But now a days it is halving time, so save your bitcoins, to get the double profit.
I will hold it until the value of Bitcoin reach a high amount and than I will sell it with a lot of profit and that should be perfect.
But it is really hard to choose the right moment to sell it on the right moment because the value is always changing.