1)What do i need to make client-KDC-server? Do i need a domain controller?
you will needs 2 systems to perform the Kerberos Server and client configuration. yes domain controller is required but in case you don't have still we can proceed with local host entire (/etc/hosts) on both system.
2)How man machines do i need? (I would make only a client, a server and a KDC).
Two machines are good go, you can make one client and kerberos server.
3)Simply, which is the first step to begin?
a) setting the host names
b) updating the date on both system through NTP
c) updating the file /etc/krb5.conf with your realm and domain name (domain name you can entry same you have given to both system like system1.test.com the domain will be test.com, make sure you use same upper and lower case in krb5.conf. And your kdc server name will kerberos server FQDN.)
d) create the database with command kdb5_util create -s -r TEST.COM
e) start the service krb5kdc
f) Add the principle users & host
g) set the ssh_config file to allow GASSAPIAUTHENTICATION anf GSSAPIDELEGATECREDENTIALS and restart the sshd service
h)perform the client configuration
i) test the user from client side.
Hope this will be helpful. In case any challenges let me know your email id I will share complete tested step by step document.