Hi, I have installed Windows 8.1 Enterprise but I don't know how to activate it. I will be very grateful, if someone can explain me that. Thank you in advance!
Your best bet is to use KMS. Head over to that section and pick the tool of your choice. https://forums.mydigitallife.net/forums/kms-tools.51/ Microsoft Toolkit has been the most popular but it is no longer being developed. However, it should continue to work just fine with Windows 8.1. KMS-VL_ALL is also quite popular. My favorite is vlmcsd (running on linux box, router, cell phone). Vlmcsd takes a little more effort to setup, but is rock solid and NEVER has a problem with anti-virus blocking activation.
FYI: There are 2 tools KMS_VL_ALL (by @abbodi1406) and KMS-VL-ALL (by @qewlpal). The latter uses vlmcsd as emulator.