to buy a wireless router - it doesn't have to be a BT model.
Then you'll need a wireless adapter for your Dell PC. That can either be an internal, PCI card type, or an external USB model.
You connect the router to the BT ADSL line, via a microfilter. You can buy these for a couple of pounds at any decent computer shop, and you'll need one for every phone extension in the house that has any kind of device connected to it - like a phone, or a fax machine. If you don't do this you'll get bad crackling on the line, and the fax won't work.
Once you've connected your router to the phone line, connect one of the computers to it using the ethernet cable that's supplied; it doesn't matter which computer you use. Connect the router to a power supply and turn on the computer.
Launch your web browser, and enter the router's address - it will be provided in the literature that comes with it. That will allow you to access the router's configuration interface where you enter your BT broadband username and password. When you've done this you'll be asked to connect, and your rouiter will establish a connection with BT broadband - you're now on the web.
You can now remove the ethernet cable connection between router and computer, and use your new wireless connection on both machines.
The router will in future establish the connection automatically, each time you switch it on. I don't recommend that you leave it on permanently, there are security risks attached to leaving a router running all the time, and it's not necessary.