Without sounding sarcastic what you can do as you haven't a clue read up about roof construction in the library try some garden design books as well as building books, then decide if you can tackle it. All so did you take note of how it was constructed ie:how were the rafters/struts fixed to the posts.
Alternative buy a new one but one with a more sound roof. Remember we all have to learn something in life and you may suprise yourself and the sceptic neighbours and friends who will be standing open mouthed when they come to see your finished roof.
I'm going to take spelling lessons as I've had to correct many words that's because I'm a novice at typing with all fingers and thumbs, sometimes all thumbs. Good luck and Happy New year.
Bob
January 2011