It has been 8 months my team is working with agile method. One of the challenge people face in agile when they come from waterfall method is story point estimation.
We use Fibonacci series for story point estimation. 1, 2, 3, 5, 8, 13 and so on. We assign a point to each story depending on complexity and development time. Higher number is more complex story and more time need to develop that one. It is hard to estimate in days but easy to give points by comparing the story with other stories. For example, if you say that to make a cup of tea you need 2 points then to cook rice you may need 3 points because making rice is more complex or time consuming than making tea. In this way you can compare stories with eachother and give them story points.