Originally posted by Eugenio Flores: how many requeriments do have to gather to start the project implementation ?
If you're doing agile, you should still have a pretty good idea of what you want to build at the project outset. Define a vision for the product. Brainstorm a good number of stories for the product. Do a quick estimation of each story. But that's usually sufficient--you need not supply exhaustive detail at this point. You want to think about just enough detail with these stories so as to be able to produce a roughly accurate initial release.
Plan out the initial release, commit to a few stories for a first iteration, and start learning how to deliver on your commitments with every iteration.