1. Make sure you read as much as possible to avoid beginner mistakes. Make you familiar with the technical aspects at least the basics. Things about safety (never share your private key and keep it safe, offline), to choose a good wallet (never store a big amount of coins at your exchange wallet). And read how to avoid phishing sites.
2. Don't trust anyone. There are many scammers out there hoping for unexperienced newbies and they hope to stole their money. If an investment opportunity sound to good to be true this is most likely a scam. Remember of Bitconnect? Lots of investors were scammed because they didn't beleive this was a scam. The most frequent errors are those you can avoid from yourself by doing your own research. If you are not sure, stay away. Somitimes not lost money is also a profit.
3. Only invest what you can afford to lose. This point seems to be the easiest but also the most challenging one. It's important to think clear even if you are very optimistic about this coin or project. It's wise to go for slow but constant profits. Don't fall for FOMO or FUD