[Edit: Was still typing while Decrepit posted; that post may be clearer than mine.]
Your question isn't clear; is there an existing wifi network in your area (your campus or apartment building, for example) or do you already have a wifi router connected to a DSL or cable modem? I'm assuming there's a wifi network already there that you don't have control of, and you just want a better connection to it.
If that's the case, then you have two choices:
1) Add a booster with a larger antenna to repeat the external signal.
2) Stick a bigger antenna on your computer.
2 is obviously cheaper, but to my knowledge no recent Mac has an external antenna port, so this isn't an option unless you're brave enough to do some physical hacking on the computer (which, if it's still under warranty, would void it).
So your only option would be to buy a router or dedicated wifi repeater with a "universal" repeater mode (meaning it will repeat any signal, rather than one from a WDS base station, which isn't going to work if you don't have control over the wifi source).
One example is the WRE54g, which I've used and frankly sucks if it's not repeating a Linksys base station. There are other dedicated repeaters and some routers that have the same feature.
One of these devices, set up properly, should grab the existing signal and strengthen it enough for your taste. If it alone isn't enough, a larger external antenna will further boost it, but it may not even be necessary since you can just locate the booster in the spot with the best reception. Be sure you pick one that has a detachable antenna, though--many (such as the WRE54g) don't.
Now, if you already have a wireless router and just want wider coverage around your house, then the answer will depend on whether the one you have supports WDS and/or has a detachable antenna connection. If it has a detachable antenna, a larger omnidirectional one could help. If not but it does have WDS, find a router that supports WDS repeating and put that toward wherever you want the stronger reception.