Colin
Welcome, my first bit of advice is , its a life long apprenticeship, you won't learn it all in one day , there are also very few absolutes in fly fishing. For Rainbows size 10 & 12 flies would be the most useful , though for dries you could go smaller depending on what they are taking.
Experiment , go up or don a size and see if that works etc
The forum is a good place to learn , just ask away. Also buy at least one of the fishing magazines , Trout Fisherman , gets a lot of bad press but is a good place to start. Also get a how to book, Stan Headley's "Loch Fishers Bible" is highly reccomeded
http://www.amazon.co.uk/Loch-Fishers-Bible-Stan-Headley/dp/0709081421/ref=sr_1_2?ie=UTF8&s=books&qid=1251827450&sr=8-2 Jim