If you've signed a joint tenancy agreement with your housemates, it's likely you'll only need one TV Licence to cover all the TV receiving equipment in your house. So it's a good idea to share the cost. If you don't have a licence between you, whoever is found watching TV when an Enforcement Officer comes to visit risks prosecution and a fine of up to £1,000* plus any legal costs and/or compensation they may be ordered to pay. It could be you, whether you own the TV or not.
If you've signed a separate tenancy agreement and you're using a TV receiving equipment in your room, you'll need a TV Licence of your own.
For further information see Tenants and lodgers.
Find out more about when you need a TV Licence.