I can see ARK is the only coin in top 20 list (except USDT) of coinmarketcap which is positive. How can I buy this coin?
You can buy ARK for example on bittrex.com in exchange for bitcoins. That means that to buy it, you'll have to purchase Bitcoin first and deposit it there.
You can buy ARK from CoinSwitch.They provide better rates for your conversion. https://www.coinswitch.co/exchange/btc/ark/0.1
You can now buy fast at Binance but you must have eth or BTC to deposid it