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 receivers 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 is at risk of being prosecuted and fined up to £1,000. 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 receiver in your room, you'll need a TV Licence of your own.
You can buy a TV Licence online.