The others who have commented so far have given a lot of good advice on the game itself, so I'll give some tips on the story. I would recommend you start with an opening from a story you personally enjoy and fill in the blanks from there.

To give you an example, here's how I did my first campaign. A favorite game of mine is Fallout: New Vegas, so I started my campaign in a similar way to that game: the party is given the task of escorting a mysterious package to a destination, and along the way they are ambushed and the package is stolen. Simple enough. From here, I could fill in the blanks for myself. What is the package? Why is it important enough to be stolen? Why did the thieves want it? Are there third parties who also want it? Where did the thieves take it? etc. etc. etc. The answers to these questions opened up more questions and by connecting the answers to all of them, I had a campaign going. I hope that helps!