Hasta La Motion!

i’m new to maya and new to bringing a cgi object to a live action footage……………so i learnt how to track motion (using boujou) and did ma first render and that worked well..BUT ma question is how can put my 3d model behind something in ma footage…i dnt know if someone uses particle illusion but init if u want to put an emitter behind somethin in ur footage u mask that thig using a tool called blocker so….in maya how can i put my 3d model behind something (not a 3d model NO…but something that is present into ma live action footage) ???

E.G :

i wanna put a model of a 3d tower behind a building in ma footage so the lower part of the tower doesn’t appear (behind the building) and the higher part is appearing

THX IN ADVANCE