What is your background in Information Technology?
if you manage to register in this platform is because you did the challenge that shows you are familiar with the concepts needed.
Start by the challenges. They are rated by difficulty. Start with the easiest ones.
For the machines, I recommend to pay for the VIP membership to have access to the retied machines. In this way you can read the write ups and try what you read in the retired machines. Then you can replicate the same methodologies in the actives ones. Sometimes you will see the same scenario in the active ones.
After that. Just investigate, read and practice.