If your house does not have a basement, go to the lowest floor and get in a small interior room or hallway. Stay away from glass and exterior walls. This is iffy at best, and a waste of precious time. Leave the windows alone and get as far away from them as you can. If you are in a mobile home, seek shelter elsewhere.
Mobile homes are notoriously unstable and are frequently destroyed during tornados.
If there is no storm shelter nearby, go outside and find a ditch or depression and lie flat, face down, with your hands on the back of your head.