Internet service providers (ISPs) usually include a WiFi modem with new broadband contracts. These modems come with all settings preconfigured so you can plug it in and start using the internet straight away.
But if you're looking for the cheapest broadband plans, you will find that many of these don't include a new modem and will require you to BYO. For the most part, WiFi modems are all pretty much the same and most should work with all ADSL2+ broadband plans (different modems are required for Cable broadband, though).
When your new ISP has finished setting up your connection they will provide you with the settings you need to input into your old modem, and guides on how to do this.
Generally, you need to connect the modem to a laptop using an ethernet cable. Then open a web browser window and put 192.168.1.1 into the address bar. You should see a box asking for your username and password. If you don't you will need to search online for the IP address of your specific modem brand and model.