Can anybody explain how to install a masternode for someone with no knowledge about that? Step by step would be very nice, then all people can do it if they want.
There are step by step guides on the CRAVE website.
is there any minimal threshold for initial purchase?
Initial purchase? No, you can get however much you want.
Staking has no minimum, though the more your have, the greater your weight, and more chance of rewards.
To run a masternode, you need 500 CRAVE.