Is there any prep work i should do before signing up for hacker rank?

What do you mean by signing up for "hacker rank?" Are you talking about the platform for online tests. At Formation we send out thousands and thousands of CodeSignals and HackerRanks assessments we have developed, and there's nothing wrong with signing up and doing their practice materials, it's a very common legitimate service. You can use their examples to practice and learn for free! They cover all kinds of topics.