1. Get familiar with IT systems first (as mentioned above) - Here you will learn the basics of systems administration and networking. Learn the difference between a switch and a router, understand subnets, play around with firewalls. One of the best things you can do is mess around with retired IT equipment to make a lab. You can pick up retired IT equipment from a lot of different places online or locally - things like old servers, switches, etc go for very cheap. Once you have this, you can test various configurations, mess around with AD, etc.
2. Learn a scripting language - This will help a lot when you start performing tests. Many times you might need to look through PowerShell scripts, Python, Bash, etc. As you learn the basics of these languages, you will be able to start developing your own scripts.
3. Learn Linux - a lot of pentesting tools run natively on Linux, so this is very important. There are Windows tools available, but Linux is more often used in pentesting. It's a very powerful operating system with a lot to offer and can be very secure from an opsec perspective.