BSp trees

Here's my problem: Im writing a game engine and i wish to implement a bsp tree for collision detection and culling. My levels will consist of a number of .x file meshes that have been positioned in my level editior. Once i have all the mesh's loaded how do i add them to a bsp tree? all the examples i have seen, seem to require a list of polygons. If i retrieve the polygons from the vertex buffer of each mesh, how do i know where to put these in the tree? Thanks in advance.