Conquer stuff. Get powerful. Do the Senate missions while you're at it, since the rewards can be useful (or not; an unit of Barbarian Cavalry...?). Watch your popularity rating among the Plebs (and chuckle at the descriptions) rise as you, quote, "bring Roman order to stinking Gauls". Once it starts approaching the top of the scale (usually accompanied by the Senate increasingly beginning to dislike you) you can start seriously planning the civil war (easy enough to trigger at that point).

Your surefire sign the time is getting ripe is if and when the Senate gives you a mission involving your faction leader committing suicide, or outlaws either or both of the other families and tells you to attack them...