So a basic, generic overview of SoapUI:
SoapUI is a tool that opens up a socket on your computer, takes the information that you put into a visual representation of the request and transmits that data to a system and then displays a visual representation of the response. That is the most basic core functionality of SoapUI.
There is several other pieces that makes SoapUI more complex and easier to use.
Sadly, to compile 1 single tutorial for SoapUI would either be far too large to be useful for anyone since it would encompass so much information that about 25% would be useful to you or it would leave out information that is useful to 75% of the other users.
The best way to go about learning SoapUI is to just use it, as you run into an issue, google it. If you can't find the answer, ask a question here (Or on
SQA.StackExchange) and someone will be of assistance (it might take a while though).