I assume you mean connect to a nearby public WiFi that is out of range. You actually can build a decently powerful antenna yourself, but you need a USB wifi dongle. Search 'cantenna' and you'll come up with lots of results. If that's not strong enough, you'll need to buy a high gain directional antenna. A yagi antenna should do the trick, they can usually be had for about $20 online. Make sure you get one for wifi, not tv or other channels!