Your Wi-Fi password will have been set by your network administrator if you're at work, you'll need to ask them.
If you're at home, there's a high chance that (as long as yourself or someone else that has access to your router) the password is printed on the bottom of the router.
If it isn't there, or is not working, call your Internet Service Provider, and they should be able to help you.
Hope this helps!
- Callum