Generally speaking, the forum is for technical support related to the SDK, not exactly for learning OPC UA (or programming) basics, that you will need to do on your own, or attend a workshop etc.

The SDK tutorials, samples + javadocs do cover some parts, for example, this is explained in the client tutorial (PDF) section 9 “Read Values”. There it is explained as:

The Attribute to read is defined by
the attributeId. Valid IDs are defined in the Attributes class. Note that different Node types (or
NodeClasses according to the OPC UA terminology) support different Attributes. For example, the
Attributes.Value attribute is only supported by the Variableand VariableTypeNodes.

Our partners documentation (for c++ SDK, but it explains the basic concepts) could be useful: http://documentation.unified-a…..bjectModel

Also it might be easier to start by using the https://www.prosysopc.com/products/opc-ua-simulation-server/ + https://www.prosysopc.com/products/opc-ua-client/ + https://www.unified-automation.com/products/development-tools/uaexpert.html as they are generic servers and clients with UI.