Hey Hitman,
The main thing is make sure they have a job, can pay the rent and have money for a deposit pay. You can get people checked out as well – for credit and tenancy history as well. As you will be there as well it’s also very important that you can get on, so be upfront with rules and your own quirks and make sure you find out theirs as well.
There’s my spareroom.com thats set up for this type of thing. Also if you get a couple you can charge more and have the other room empty – or even rent them both so they can use one as a dressing room/lounge.
There are many ways to rent a room – to start ask around your friends as they might know someone who needs a room.
Hope this helps.