A starting point, to introduce oneself to the main CTF categories.
Hacking websites.
The art of exploiting a computer program on the lower level.
Cryptography! Breaking old and modern ciphers.
Understanding how programs work by analyzing their code and behavior.
Investigating and understanding what has happened on a computer system, preferably before the hackers have escaped.
Finding and exploiting the bugs that were never meant to be found.
Challenges that don't fit in the other categories.
Explore the weird behavior of hardware!