not sure about windows phones but on android its right in the wifi options area. should be in a similar location for a windows phone i would think. the default on my phone is no password not sure if its the same on yours.
if you meant for your home router... you should have this information written down. if not you might be able to pull it off of a device you currently can connect to the network with under networking options IF it lets you copy and paste the code since passcodes are normally shown as ********.