how do i make a model solid?

I am currently tryin to create a horror game and i have created a mantion in blender but i cannot seem to make the walls and floors solid.. I first experienced a problem being able to see through some of the walls so i reverted back to blender and flipped the faces and that problem has been fixed but im having trouble making everything solid so that im no longer able to walk through walls... ANY and ALL help given would be very much appriciated as one comp. tech to another and i thank you for your time