Took me about 6 weeks to reach elite hacker rank here by doing only easy, medium and the easiest of hard linux boxes. Did some challenges here and there whenever i was missing 1-4% to reach the next rank.
Several times i had to ask for pointers and in some rare cases where i was unaware of a technique or exploit i had to straight up ask for the name of the technique so i could read on it.
Start with the basics.
-Linux Command Line by William E. Shotts. Follow through with the examples and familiriaze your self with the OS.
-How linux works, what every superuser should know.
Read casually to get an idea of how things work
-Comptia Networking+ is also a great read
-Possibly Mastering Ubuntu Server by Packt publishing
Then
-Watch ippsec youtube channel. Start with the easy boxes. Great practices and procedures for htb.
-If you need to see how to enumerate something google “ippsec search” and enter your keyword there.
-g0tmi1k is a great blog. Read from oldest to newest.
-Vulnhub boxes are way easier than htb
-John Hammond has also a great youtube channel doing challenges in other platforms that are probably easier for you.
-You dont need to be a master programmer but very basic knowledge of python, curl, php, js,sql and bash will help you a lot. Send requests with curl and python and read the responces, identify vulnerable php code, understand how and why sql injections work through php, simple bruteforcers in python etc.
I also dont generally like the challenges.
I prefer the boxes marked as “Real Life” since the mistakes and techniques used there are closer to real world scenarios
You dont need to know everything by heart in the beggining. Just knowing that something is possible and going back to read on it whenever you think you might need it is enough.
Also, keep notes. A LOT of notes!