OK, I've got the full wifi set up at home with laptops for me and the missus, as well as a stand-alone hard drive, a desktop computer, a networked printer, and an extra laptop hooked into the stereo for playing MP3s. I've played around with this stuff quite a bit.
You have 2 options.
Option 1: Buy a combination adsl modem and wifi router. These guys are usually more expensive. Probably in the 40-50 pound range.
Option 2: Get a separate ADSL modem and a wifi router. As Super Pollack mentioned, your DSL internet provider should either a) provide a modem or b) get you one really cheap. Don't get a USB model. Go for one with an ethernet port. Ethernet ports are the plugs that look like a phone cord only much wider. They're what you use for wired networking and since your router won't be plugged into your computer most of the time, a USB plug is useless.
Then get a wireless router. I highly recommend Netgear. I've got a Netgear Rangemax wireless router WPN824 and it works like a dream. Do NOT buy D-Link. They're generally cheaper but glitchy. My first wifi router was D-Link and it stopped working after about a year. I've heard mixed reports on Belkin routers. Linksys, from what I've read, seem to be ok. These should cost around 25 pounds.
Make sure the router or router/modem combo you choose supports 802.11b/g wifi. The 'b' and 'g' are the most widespread flavors of wifi and probably what you've got in your laptop. There are also 'a' and 'n'. 'a' never quite caught on. 'n' is the newest, fastest version but the specifications haven't been approved yet so anything you find is based on manufacturers guessing what the final specs will be.
Once you've chosen a router or modem/router, you plug it in and hook it into your modem with the ethernet cable that will come in the box of one or the other. Easy enough. Then you run another ethernet cable between your laptop and the router to manage the setup. Your router instructions will give you a web address to type into trusty old internet explorer, which is really just the local network address of the router. This pulls up a page where there will be some sort of wizard to guide you through the setup process, after which you will be able to use the laptop with the wifi (without the ethernet cable).
Check out cnet.com for some reviews of various routers. They also have a good guide to setting up a wireless home network.
http://reviews.cnet.com/Networking/2001-3243_7-0.html?tag=cnetfd.dir
Hope that helps
- seems much easier that way 
(to be honest I've been stealing theirs all day 
