It depends what you mean by learning kali.
To learn command line you could follow any decent books, youtube tutorials and linux.org tutorials.
but if your wanting to learn how to use the penetration tools tools in kali thats a slightly different matter, last i eharrd there was about 600 tools in Kali! you will want to take some online courses on nmap(netowrk scanning) and wireshark (packet analyzer/sniffer)
Id recommend getting well grounded in the linux commandline basics first, moving ,renaming, copying and delteing files, editing text files, some basic system process commands and network commands. Then that will make learning Kali tools or any linux tools easier learning curve.
I also know there is an official kali course im not sure of the cost, but the official course is likely very comprehensive