I need the wep - Linksys Wireless-G WRT54G Router

I assume you are referring to the WEP encryption key for your wireless network?
By default any Linksys router when purchased new will come "out of the box" with no security (WEP encryption) enabled. If there router has been setup by someone with the WEP key you have a few options to connect to the wireless network.
1. Ask the person who setup the router.
2. Reset the router to factory default settings. (if someone specifically set up this router, you may lose important settings) Find the labeled "Reset" on the back of the modem and press (with a pin/pen tip) for longer than 30 seconds. Then log into the router using the instructions below to add a new key or leave the wireless on with no encryption (NOT recommended)
3. Log into a factory default linksys router (using a wired ethernet connection, not wireless) by opening up your web browser to the address http://192.168.1.1 The default username is either left blank or "admin" and the password is "admin". Navigate to the Wireless Security settings and their you can view/change the encryption key. Note that even if the router has encryption enabled the default address and password may apply.
I hope this helps,

You can find this out by connecting to your linksys router with a cat 5 cable.
in your browser such as internet explorer or firefox type
192.168.1.1
login with your login/pass that you assigned to it.
go to the wireless tab and go to security
it will show the the WEP key that is needed in order for your laptop or wireless devices to connect.
BTW I would recommended using WPA as WEP is not secured.