Lots of people offer a room or more in their own home.
What you can't do is not provide a guest with their own space in the fridge or space for their clothes.
At the very least you should minimise what you leave in the fridge and make sure your guests has wardrobe and drawer space for their things.
Just having a place to sleep is definitely not all that matters. You get rated on your cleanliness, communication, location, check in, facilities etc.
Why not read the information on Airbnb's website on what's involved in hosting.