Take your time in both learning and hacking. There are a lot of important concepts you need to understand and consider in order to avoid damaging a system or facing legal consequences.
Refrain from touching systems you don't own unless you have a legal agreement which permits you to. Set up your own environment or use one which is explicitly designed for practicing techniques.
Use tools, exploits, and guides from trusted sources. Malicious tools and exploits which are designed to compromise your system or publicly humiliate you do exist and are falsely advertised to trick people.
Segregate and segment appropriately to ensure important data and systems are separate and protected from your test environment.
Test exploits and tools in a safe, controlled environment to ensure they work as expected. This can help to gain further understanding of what the exploit or tool does and artifacts it may generate.
Exploit smartly, not blindly. Understand how an exploit works and what it does before trying it. Modify it as needed to suit your purposes.
Don't use your skills maliciously. However tempting it may be, just don't. It may cost you an absurd amount of money and prison time.
A repository of ethical hacking guidelines and resources