The key is to ensure every item on your board (hope you have a board) is actionable and deliverable by the end of your sprint. If a task is missing requirements or artifacts - move it back. If a task can not result it a fully tested and deployable piece of software - break it down.

And lastly, human beings suck at estimating time - base your first sprint on hours or timeframes and then move to a point based system.

Good luck