The government's 'Rent a Room' scheme allows you to receive up to £4,250 a year tax-free for letting a room in your main residence.
It doesn't matter how much your other earnings are. If your rental income is less than £4,250 per year you do not have to inform HMRC and you do not need to complete a self assessment tax return, unless you already do so for your other income.
If the total amount of income you receive is more than £4,250, you have to declare the full amount by self assessment, and pay tax on any income above that if you've used up all of your allowance.
You can find more information about the 'Rent a Room' scheme at direct.gov.uk.
Bear in mind though that you may need permission from your mortgage provider to let out a room in your home (if you have a mortgage) or the landlord if you are a tenant yourself.
Taking in a lodger can also affect your home insurance so it's worth looking into those two aspects of letting out a room as well.