A quick
for "lodger agreement" brings up quite a few downloadable suggestions.
Check your local newspaper for going rates. It's probably one of the best places to advertise. Other than that if there is a major emplyer nearby, sometimes their HR depts may try and assist employees looking for accommodation. I'd steer clear of agencies though, most of your profits would go on fees.
Check on your tax regulations. Most of the time the amount you earn won't affect taxable allowances, but if you get any tax credits or other stuff, it might offset some? Council Tax, if you currently get a single occupier discount, you will loose that entitlement.
Check your home and contents insurance policy, would they cover you for any losses. Do they need specific notice?