Start reading from Service Now Wiki.Start from get "Get Started".What ever you learn you can practice on demo instances provided by Service Now "https://demo.service-now.com" . In the this link add "001" to "023" after "demo"
It's simple o understand and start from the above.
2.Once you are familiar with the basics then you can go ahead with servicenowguru.com.It has a lot of useful code snippets.