I started my PhD last year in a field slightly removed from my original degree. The first thing I did was spend two months working from cover to cover through a text book about the new topic. It really helped me, so I'd suggest trying something similar. Find a book like "introductory whatever you want to learn" and go through it doing all the examples and everything. It's really boring, you feel like your wasting time, but when you've finished you realise how useful it actually was.