There is a distinction between wireless networking and wireless Internet connections. Actual wireless Internet service would be provided by a cell phone or satellite company, and you'd just get a little USB receiver to plug into the laptop.
Assuming you really meant wireless networking, you'd need to get a
wireless router (sometimes you can get a modem + router combo box from your ISP, but they always suck). Good brands of Wifi gear include Linksys and Netgear, and exclude D-link, Trendnet, Belkin, and other random general gear/hardware vendors. Before picking up parts, there are some questions to ask:
1) Does your laptop have any sort of built-in Wifi network adapter? If so, what kind? 802.11 B, G, or N?
If your laptop has a B or G adapter, all you need is a 802.11g router. If it has a N adapter, you can still get away with a cheaper 802.11g router, and it will function in G mode.
2) If your laptop has nothing, you will need to get both an adapter and a router. At this point, you need to choose whether you want to go with 802.11 G or N (B is obsolete and noone makes just B gear anymore). N provides better networking speeds and better range, but will cost more money (and it has the dubious position of still being a draft standards specification..). The speed will only matter in network connections between your computers, since
802.11g is likely faster than your Cable or DSL service (19 Mb/s throughput).
Wifi adapters for your laptop come in both USB and PCMCIA slot flavors. USB can be used in any machine, but sticks out the side. PCMCIA cards fit mostly inside the machine, and are thus less obtrusive. You cannot use them in general machines though, since most non-laptop computers have no PCMCIA slots.
Assuming you don't go with cheapo gear, you're looking at spending around $100 US on a 802.11g router + adapter (more if you go the 802.11n route).
=0=
Regarding setup, the easiest thing to do will be to plug the uplink port of your new router into the current modem, then plug all ethernet cables into the new router. This way, all of your computers will get IP addresses in the same network range, and life will be much, much easier. It also helps to set up a rule in the modem to blindly forward all traffic to the router, and just use the firewall stuff in the router itself. This way, you don't have to worry about forwarding ports from the modem to the router, then from the router to the computer.