The only way to know when is the good time to buy and sell is to monitor the market price value. In that way you will know when to buy, if you need to hold and when you to sell your coins. Just like few months ago market price down but these days its slightly increasing again.
In my honest point of view, the longer someone will wait and take time in making decision, when is the right time to become a part of crypto world, the more will be the charges of joining the party. Bitcoin is becoming expensive with every passing day. It is also clear that this coin has a bright future and can become worth 40k dollars by the end of this year, so what is the logic behind wasting time?