If you look into metasploits free course, there is a wealth of knowledge to be learned there. It allows you to download a free vulnerable virtual machine that you can practice on while going through the course!
You can of course take on the challenges here at hackthissite, which will also teach you a whole lot about hacking as well. In addition, there are tons of CTF challenges that go on that you can participate in to try out some of your new tactics! You have vulerable .iso files you can download and attempt to hack into. You can build web applications to exploit on your own! There is a variety of things you can do to help understand hacking better.
A real question would be, what do you want to learn how to hack?
Websites, Computers, Applications, Reverse Engineer Software (to bypass things like trial periods), Wifi, etc, etc
If I had to tell you to choose one, go with the metasploit course, and download metasploitable to test out your skills!
I don't care how ‘secure’ your systems are. If you have stupid people running them and using them, you can't win.
For those that know