Most, but not all, Broadband ISPs supply a 'free' USB (non-wireless) modem and a few filters, when you first sign up. For a wireless set-up on a Broadband enabled BT telephone line, you will need an ADSL wireless modem/router - some ISPs can supply a limited choice at extra cost, or you can buy one elsewhere.
You can read
some reviews here. I've used a Netgear DG834G for about six months, and it's been very reliable.