I cant figure out how to to go up steps and walk through a house, i just keep walking through it

Im using unity 5.2 for starters. I imported a asset from the store, which was a building bundle. but when i put the building in the scene, i cant go up the steps or walk through the house, i just keep walking through the structure as if it wasnt there. i know theres a box collider and a mesh collider and all sorts of things that i have to do, but i dont know the steps to do it. Im fairly new to Unity, so to be honest i have no clue what im doing. Any helpwouldbe greatly appreciated