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I'd recommend trying to get a copy of C&H on loan from your library - they might be able to get hold of it from somewhere, you never know. It is the kind of book you read but don't refer to much though because it is a bit out of date and doesn't help much with identifying flies.

As Tony said, the order is so huge that you have to usually start with a key to families - get familiar with the different groups and things will make a lot more sense later. Then dive into one of the easier groups - like hoverflies (Syrphidae) and avoid the nastier ones like the plague ;) There are sometimes free keys to some groups so just ask here and you might be lucky :)

Basically, it's a fascinating group of insects to study but you have to do quite a bit of asking-around for help & guidance (hence this forum) and most groups will require specimens/microscopes to go into any great detail :)