If your house has a basement then it's best to retreat to the basement in the event of a tornado.
The safest place to be is below ground during a tornado. But if you don't have a basement then go to a room that has no windows such as a bathroom if the bathroom doesn't have windows. Or any other room that has no windows.
Go into the center of a room since corners of rooms tend to attract debris. If you can get under a mattress do so as the mattress can help protect your body from debris falling and hitting you.
If you can though it's best to leave the house and go to a shelter. Check with your town as most towns and cities should have a public tornado shelter you can retreat to if safe to do so.