Improve Your Skillz
One of my favorite CTF sites. There's so much to do and learn.
https://www.root-me.org
https://www.hackthebox.eu
https://www.endgame.com/blog/technical-blog/how-get-started-ctf
http://liveoverflow.com/index.html
http://captf.com/practice-ctf/
http://scanme.nmap.org/
http://roothack.org/