How many computers do you have? Are you trying to hook up broadband internet only don't have a connection point near your computer? Or is this for Xbox or PS2? We are having a hard time figuring out what you want to do. Without two computers that need to "share" either files or an internet connection, or a console system, what is the point? PLEASE tell us what you're wanting to do. You said you want the antenna outside, are you trying to set up a network with, say, a friends computer "down the street" or something???
Ok, from what I've re-read it looks like you're looking to pick up a wifi signal from someone else's router? If that's the case, you can do it with a wireless network card, however, the range will be at best 1000 ft with an outdoor antenna. I imagine you can buy one but it won't be cheap. If that's the case, look
here for some outdoor wifi antennas. Look at a directional antenna if you're going a pretty good distance, especially if there's a decent line of sight to the source. Yagi or parabollic would be a good fit for max signal transmission. Also, you might need an external antenna at both the transmit and receive stations....
What are you trying to connect to? Got a friend down the street or coffee shop with free net access or something?? If you're just looking to set up an external antenna and "see" what wireless networks are around, you may find some unsecured, may not, may not have any access points around at all. Either way that wouldn't be advisable.