Get to WHSmith and get the most recent issue of Pilot magazine, there is a guide to becoming a professional pilot and will tell you everything you need to know as a first port of call.
Oh, and don't waste your money on a Class 1 medical just yet, they are only valid for 1 year before needing renewal, and you're too young to hold the commercial licence for which it is needed. By all means consider your medical fitness early on, though.