Debian has a really excellent wiki on setting up your wifi:
I have personally used network-manager, wicd, and ceni on Debian Xfce. They all worked perfectly fine once properly configured. Network manager is the recommended and default for Debian so why not start there? If you patiently follow the steps in the link above, ask specific questions when you get stuck, and provide more details (such as what is your wireless card chipset?) then I'm sure we can help you.
At the risk of stating the obvious, check that your password is correct and your caps lock isn't on when you type it.