I now have $ 500 in capital. Is this the right time to hold the coin. and what kind of coin should I hold? I need some advice now, Because according to my judgment this will be the momentum for altcoin to explode in the end of 2018
Actually, the thing that you are going to do when the market price goes down is to currently hold your coin or buy another coin then hold it. Sell your coins until the price comes back so that you can get some money.
That's what I call a healthy speculation. You're absolutely correct. We need to buy now to sell at a profit later, and we need to hold better coins and wait for them to moon to make even bigger profit in longer term.