Like the Chinese proverb - "The best time to plant a tree was 20 years ago. The second best time is now.", the best time to buy a bitcoin was 4 years ago. The second best time is now. Don't worry or regret when price go down after you bought because if you see the trend, the price will go up. Buy bitcoin as a long term investment. And like the author mentioned, never keep your coins in an exchange. Keep in multi-sig paper wallet or hardware wallet.