Give me siege tips

[Bozarius]

I am a novice player and I am playing vs the computer. When I get the advantage on one guy, another wages war with me. When I get the advantage on them, yet another does. I am playing on marignon on the map with all of the other civs. First Caelum waged war and I had the chokehold on them. And right as I was getting ready to finish them Jotunheim waged war. Then I got the chokehold on them and Ulm is now starting to attack me. Problem is I got tons of forces bottled up trying to siege both Caelum and Jotunheim. Anytime I storm them I get my butt kicked. They never seem to starve even after fifteen turns and are armed to the teeth when I get in there. I have had all surrounding provinces when I rushed them. Is the only way to do this properly to starve out their dominion?