Depends on the kind of encryption, password, etc.
You could sit there and try lots of passwords (note: this is probably illegal).
From a purely theoritical viewpoint, wifi should: be encrypted, have strong passwords. So, if you want to break that, you'll need to find out how to a) work out the encryption, b) break said encryption, c) break password.
A few places to start looking:
google. Google is *the* place to be (and one of my few friends, so be nice to her. Yes, google is a her.) You'll need to know what to search, tho.. try 'cracking', 'wireless hacking', 'wireless sniffing'. http://en.wikipedia.org/wiki/Wireless_security
:: To break *anything*, you should know *how* that thing works. Learn. Go and read about wireless, how it works, how users are authenticated. Learn how the data is sent from a to b. Wikipedia is a good place to start (and dont forget the links at the bottom - they are a good place to go after youve read wiki
Try looking for pentesting. There are lots of places that will help you secure networks, and wifi pentesting (that is, deliberatly trying to penetrate your own network to discover its vunerabilities so you can fix them) is a fairly well discusses area, if you know where to go.
I think what you are asking is very similar to the question "how do i wardrive? What tools do i use?" Try finding the answers to that in google.
Some more things you might want to look at ::
Mac address / ip address spoofing
Please note, I dont think that using any of this information, slight as it may be, to break other peoples networks is a good (or leagal) idea. Use it for your own network to test and harden it, and for purely educational purposes.