Noticing there were a few new names around I was giving some tips in chat when asked to put them here because chat scrolls so fast. These are not intended to be a detailed guide, just some very basics that beginners may not know.
1) Do the Tutorial. Not only does it give you some free practice with siege weapons, but it gives you 2 skill points, at least one of which should be spent on an Alliance War skill. Siege Shields would be my recommendation because they are very useful, and there are rarely enough. You also get a Limited Forward Camp at some point around now. Save it. (Please note that if you missed the Tutorial and have got to Rank 2 Alliance War you won't gain by going back to do the tutorial, since what happens is you gain the first 2 ranks by doing the tutorial as far as I know. You might get the camp but it's pretty awful as it only allows about 5 respawns.)
2) Stone Trebuchets and Faction Ballistas are best against Walls and Gates. Meatbag Catapults are very good anti-personnel weapons. Fire Ballistas are very good against siege weapons and players. There is a weirdness in that it seems when shooting at a Postern Gate, you should not shoot the door itself, but the wall just next to it (bugged iirc). Siege Weapons can be bought from Siege Merchants who are found inside Keeps near the Transit Point and at Resources (Mines, Farms and Lumber mills)
3) Oil Pots need a long drop to work (at least 6 metres). At present it seems they are only good for defending the walls of Inner Keeps and Flag Rooms in Keeps. (Each Keep has 2 flags in adjacent rooms. To capture a Keep you need to capture both by having friendly players close to them for a period of time). Oh, and Oil Pots are the cheapest siege weapons you can buy at 800 Alliance Points.
4) Rams are specialized siege equipment for attacking main gates. They need 6 people to man them and these people need to be close to the ram to affect it. Ideally they need to be inside the ram - I think you get an extra 10% effect for being inside holding block, but I'm a bit hazy on the exact details.
5) There are a LOT of Vampires in Cyrodiil. Give serious thought to your Fighters Guild skill line.
6) Get a Horse. Cyrodiil is big. At present everyone thinks they need maximum speed horses but these make you very vulnerable to being ambushed. It might be that some stamina would be of benefit.
That's a start anyway. Hope you find the above points useful. More experienced players, please feel free to add your own tips.
(Edited for Update 5)
Edited by Thudunblundur on November 5, 2014 10:57AM