Research your area to see what other landlords are asking for their room rentals. This will allow you to gauge what a fair rent amount would be for your area. Keep in mind; asking far above the rental rates for your area will slow down your rental process.
Do you require a security deposit? If so, how much.
Decide whether you will include utilities in the rent you will ask. If not, do the math and estimate what the renter utilities will be, so when asked you can provide the potential roommates realistic rates.
Will your rental be for an individual only, or would you be willing to accept a couple or even multiple roommates if you have a large accommodation? If you are willing to accept more than one person, how much more will you charge, if you will require a higher rent.
Do you accept none caged pets, like cats or dogs? Do you accept contained pets like birds or fish?
What is the minimum/maximum time range you are looking to have a renter for?
Are you offering the room furnished or unfurnished, or will the renter have the option? If so, how much more will the rent be with furniture?
Will you accept a smoker indoors/outdoors only?
Will the renter have access to the common spaces of the home (kitchen, living room,etc..)
Have a standardized lease on hand that incorporates all the key points above. It is very important to have a signed lease that both parties have agreed upon to avoid any possible situations in the future.