SOA is about design principles (some might even say it's a paradigm) and less about tools and implementation, so it is difficult to fire up your IDE and "implement an SOA".
Because SOA is bigger and complexer than, say, a design pattern, most sources on the subject only deal with the theory and do not discuss implementation details.
If you understand the concept and know Java, then you can start implementing on top of existing frameworks (see other answers), by reusing existing components, or, if you are feeling I-want-to-write-a-C-compiler-from-scratch-y, you can try to design and code your very own SOA from the ground up.