Buy in a cheap price and keep on holding then see what happen in the market, its hard to know if the coin you buy really increase, all you can do is research on it so you will know the information about the coin you have.
Even you can use this forum to know when to sell and what price is right to sell and when the price of bitcoin pump and when the price of bitcoin is dump.All you come to know by this forum itself. Even you can try of Coin desk, to know some more profit. Finally, you can sell at any price, if it's OK for you. If you brought at 8000$ and 8400$ means, you can sell it. Or if you want some more profit means, wait till it reaches 9000$.Selling is purely based on your wish.