Models can be walked into unless they have colision around them. Colision is the clip tool. There are various types of clip tools depending on the surface you want to make collidable. For example if your surface is a metal surface, you will cover this model with clip_metal or something. For wooden surfaces clip_nosight is the one you need. Those will define the footstep sound when players walk on the model surface. I think all models are ready in your prefabs. try find map_source/prefabs/misc_models. There you will find most of the games models with clip around them. So you have to use all models as prefab. Bushes and trees usually don't need clipping. You can mess around with this and learn how it works. You can also use this to block paths or ways that you don't want your player to go through.
GL.