How do you conquer rome on rome total war

To do this you need almost full "people" popularity, you can check how much of this you have by looking at the popularity meter on the the right side of the the senate floor scroll, in order to have alot of this you need to have conquered many countries.

Edit:

An alternative to this is waiting until late in the game at which point the Senate may send a mission calling for your faction leader to "Commit Suicide". If you decline this option and let your faction leader live, a civil war will spark automatically and you will be free to invade Rome then.