Hello, Elisa and welcome to the wonderful world of Jasager!
I would suggest that you take a look here: http://hak5.org/hack/pineapple-phishing
And if you need to know about dns spoofing begin with: http://en.wikipedia.org/wiki/DNS_spoofing
There is a lot of information from the fantastic contributors on this forum. Try looking around and see what you might find. Using the search-function can save you from reading it all.
It is always good to have some basic knowledge before attempting to start phishing those passwords for educational purposes. If you are able to build your own pineapple I am certain you will learn how to use it!
And please remember that backtrack and jasager are no toys. Chances are that if you run around using it without conceptual understanding, you could go to jail or someone else could get hurt.
h**p://www.youtube.com/watch?v=74-WSM0xTyE
:-)