Theres two ways to do this - like everything the easy way and the hard way. The easy way would be to get someone with experience involved, a mentor / consultant / trainer etc, have them learn your work situation and provide the advise you need. The hard way is to read a book, then jump in a do it, get some of it right, make some mistakes, recognize and learn from them, and refine things next time around (sounds like you could use Scrum to learn Scrum).
I'm a hard way kind of guy - all you can do is back yourself, accept you won't get it right first time, but will learn nothing from not starting, and get on with it. You will probably learn numerous bad habits on long the way, so where possible use a mentor (and Q/A sites like this) - and be open to criticism and self criticism.
Referring to the OP - it's not easy to write user stories - you are correct - it's not easy, if it was, then Scrum would not be around and waterfall would be just fine. No one claimed scrum makes it easy (at least, no one who knows what they are talking about). Software development is hard, Scrum is a a way to manage the hardness, maybe making it easier or as easy as possible through better management. Scrum cannot make things "easy".