Q: what must i do to extend my existing WIFI network with my airport express ?

but i don't know what to do to extend the wifi network

The first requirement to be able to configure the AirPort Express to "extend" your wireless network....using wireless only....would be to have an Apple router providing your main wireless network.

Is that the case?

If yes, what Apple router do you have?  If you are not sure, check the model number on the label on the bottom of the router.

I have an Apple Extreme router that is "n."  The Express is also "n".

Follow Apple's step by step instructions to "extend a wireless network" if that is your desire.

http://support.apple.com/kb/HT4259

I don't think I have an apple router, I have whatever came with verizon a Westell 7500.  Can I still use the airport express to extend?