Firstly let me say that functional testing is not a standalone area. It is a part of Software Testing as a whole. I am not aware of any dedicated training courses targeted at funcitonal testing alone. However I'd recommend that you conduct a search on this site first and then on the net to get as much information. Helpfull keyowrd would be Software Testing. Software Testing Techniques, Functional testing etc. There is also a dedicated forum for training, further down on the Home page or else click here QATraining.
In the mean time there are some things that you can start working on right away, which will only help you in future. You will find a lot of information about software testing at these centers ...
From what I understand, there is a lack of formal techniques on how to do systems-level (i.e., functional) testing.
WHERE TO START? Look at what is most important. In functional testing we can not test everything - but it is most important that we test the essential. And that we let an understanding of the essential be our framework within which we "pigeon hole" all other test cases.
I myself am a great data-flow-diagram. My saying is: "It is only by dying (i.e., following the flow of data) that we are borne again (i.e., come to understand the essential functions and how they all interrelate - which drives effective functional test case planning.)