If it was a 'locked' phone, i.e. one tied to a particular service provider (e.g. EE, Vodafone, etc), it will only work with a SIM from that provider unless you pay to get it unlocked. Tell us which provider it was and we'll find you a link to tell you how to get a new SIM.
If it was purchased as a 'SIM-free' phone, you can use
any SIM in it; you just need to choose which service provider you'd like to use. e.g. if you decide to use Asda Mobile, you can normally buy a SIM in an Asda store for (I think) £1 or order one online totally free. However Asda Mobile is currently changing networks, so you can't get hold of a SIM from them right now. GiffGaff though, which is also very popular among ABers, will let you order one for free right now:
https://www.giffgaff.com/free-sim-cards