On my Belkin router, using the browser to go to
http://192.168.2.1 gets me to the router's config page.
You'll have to log in to get to the next bit though. The default password is blank. If you have changed it, you'll have to enter it and if you can't remember it your only option is to reset the router to factory defaults and start setting everything up from scratch again.
So, assuming you do remember what you changed the default password to ...
On the menu down the left hand side of the page, under Wireless, there is an entry for Security. Click on that and it will bring up the page for setting the encryption type and pass key. On my router there is a check box on that page labelled Obscure PSK which should be checked by default. Click on it and you'll see the passkey.
If the checkbox isn't there and there is nothing obvious on the page which is going to let you see the passkey, you can enter another one and apply the changes (after writing it down, and remembering that it is case sensitive !).
The pages on your router may differ to mine, but I think you'll be able to figure out what you need to click on to get there.