A good housemate ad has six types of information. It should list the location and the rent. It should also include a description of the room for rent and the larger house, something about yourself as a housemate, a description of what you are looking for in a housemate and contact information.
The location and the rent should be easy. The descriptions take a bit more thought and care with the words you use. You want to make it sound attractive and you need to be honest.
If you’ve done spent time thinking about who you are as a housemate and what you want in a housemate, it is much, much easier to write your descriptions. If you are a neat-freak, you describe that. You want a housemate for whom living with a neat-freak is a good thing! If the room for rent is small you say so and also something interesting. For instance, I used to rent a room in my house that was tiny. But is was also light-filled and the house was close to public transportation. Those benefits outweighed the smallness of the room.
The usual sort of things to list include ages of current housemates, sex, occupations (nine to five? or other schedule) attractive features of house/location/room, and something interesting that would signal to a reader that you would be a good housemate for them. Look at other postings to get good ideas but remember that you are a unique person. Communicate that.