There are two major ways to know when to buy or sell your a cryptocurrency. You can either follow the news, but be sure that you have the right interpretation of the news before doing that. I don't follow this style even though it is effective because it is deceptive at times. Many news has failed in their right interpretations or raised a false alarm, and the news impact might be deceptive. The second option which is my preferred way to know when to buy or sell a cryptocurrency is to use technical analysis, where chart patterns would guide you on what to do. You will sell when a peak/high is established or buy when a low/bottom has been established. You can also buy or sell after a breakout, and simple trading tools like price action, trendline etc. could effectively guide you in this regard.