To help dispel your worries, first of all take note of how your prospect speaks. Can they string a sentence together so that it is logical and intelligent. Take note of their dress - are they clean and tidy? These first two items may tell you all you want to know to take your initial decision 'yes' or 'no'.
Then proceed as follows:
Make yourself a checklist of all the questions you wish to ask.
Are they in full-time employment?
What is their maritial status?
Do they have any illnesses or disabilities?
Do they have dependents?
Do they want to bring any pets with them?
Do they like to play music a lot?
Do they have transport - this is particularly important if you are not near a main transport route.
Do they have any motoring or criminal convictions?
Do they drink? Or hopefully not - do they look as though they are on drugs.
How long do they want the room for?
If this list seems like being a nosey parker, remember this person wants to come and live in your home, so it is up to you to protect your best interests and those of your family or be sorry later on.
Regarding rent. You will need to issue a rent book and make accurate records of all payments your renter makes you. Do not allow them to get in arrears or you may find yourself on a slippery slope downards.
You must be firm, fair and friendly.
I hope this helps. With best wishes for any venture you undertake.