See How to Use CMD for more information on the Windows Command Prompt. See How to Get Familiar with Ubuntu Commands for details on the Ubuntu (Linux) terminal.

You may be able to take classes on networking at your school or local community college.

You can use virtual machines to install any operating system on your computer. This will allow you to test hacks tailored to specific operating systems. See How to Install VirtualBox for information on getting started with virtual machines for free.

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Avoid being associated with illegal behavior as well. If your hacking community starts performing attacks, you can be considered guilty by association. If you become uncomfortable with the actions of others, distance yourself as much as possible.