Some things to think about while deciding where to begin your tale:
1. You want to hook the reader, so some kind of action, suspense, or hint of the conflict to come is good.
2. You want to introduce the reader to your characters and setting, so some kind of scene that tells us about those things are good.

If you're still stuck, have you tried the brainstorming techniques or using the prompt generator? Mind maps are great for getting unstuck - write a word in the middle of a piece of paper, circle it, and then write as many related words around it as fast as you can. Continue to spiderweb out with word association until something grabs you. I don't have much more to suggest without knowing about your plot.

Good luck!