Just like other Battlegrounds, organization goes a long way. Unlike other Battlegrounds, however, you won't automatically be grouped into a raid, so a lot of the time players will be looking to join a group when the battles start. As long as you know what you're supposed to do, you should be fine, though. The good thing about the zone is that the map will show you where battles are and where vehicle movement is taking place, so even if you're going solo, you'll have an idea of where to head for some action.
Generally, an unorganized offense will have an easier time winning the game against an unorganized defense. It's just the way the game is built. Offense is designed to have a slight advantage in order to have the zone change control frequently. This is why Patch 3.0.8 will introduce a buff called Tower Control, which grants the offense a 15% damage increase as long as the three towers in the South remain intact.
If you took the erratic now-you-see-it-now-you-don't portal from Dalaran, you should zone in the West landing camp if you're Horde, and in the East landing camp if you're Alliance. Otherwise, you can fly in. In the first battle after a faction has had control of Wintergrasp for one period, the East and West workshops will begin under the control of defense. One of the first things to do is to take control of a workshop -- this works exactly like controlling a tower in Eye of the Storm -- so make your way to one so that control shifts over to your faction once the battle begins. If a faction has had control of Wintergrasp for too long, the battle will start with the East and West workshops under control of the offense.
Once you've taken control of either the East or West workshop, preferrably both, you should go out and kill members of the opposing faction to rank up. You can also rank up from the continuously spawing NPCs throughout the map. In Patch 3.0.8, it takes five kills to move from Recruit to Corporal, and another ten kills to rank up from Corporal to First Lieutenant. If you've got rank, pick up a vehicle and start making your way to the fortress.
If you are ranged DPS, you have one of the most important responsibilities in offense: take out turrets. Fortress cannons are the most potent defense Wintergrasp Fortress has, and they will make short work of your siege vehicles even before they get to the wals. This makes it imperative to destroy the cannons, something only ranged DPS can do. Coordinate with your raid or faction to find out which side they are attacking. Destroying all the cannons on one side makes the fortress extremely vulnerable, I can't stress the importance of destroying turrets.
When using siege vehicles, always move as a group, especially if you're only piloting a Catapult. Players will easily destroy a vehicle as they're simply not as well equipped to deal with personnel. A Catapult's Flame Breath deals good damage against players, but players can very easily outmaneuver the vehicle. Moving as a group will also allow you to concentrate fire on one spot, which is critical because offense is running on a timer.
If you are solo, the best vehicle option is the Demolisher. It has a long range siege attack as well as a battering ram, either of which can be spammed while the other is on cooldown. It also moves relatively fast and thus can get to the fortress sooner. Unless you have a gunner, do not pick up a siege tank. Half of your vehicle will be useless, you will be moving too slowly, and you will be extremely susceptible to player assault.
Offense is rather straightforward unless the defense mounts a counteroffensive, in which case it's important to maintain control of the workshops and defend workshops and towers. If you have no vehicle, protect your faction's. Ride or run alongside one to keep them intact until they get to the walls or gates.
Once you've broken through the fortress walls, some players on vehicles should take the time to destroy the workshops inside Wintergrasp Fortress. It counts towards the Achievement Industrial Warfare, which only counts if you're piloting the vehicle. It will also curb the defense's ability to produce siege vehicles, which will help your side a lot. If the main doors of Wintergrasp Keep are broken through, all efforts must be focused at getting into the room. Riding vehicles into the room is a good strategy because players cannot be targeted while inside a vehicle.
Inside the keep, tagging the Titan Relic shouldn't be a problem. If the door is broken down with ten minutes or so to spare, it's game over for defense. In the ensuing chaos, it should be easy enough to capture. Defense will be spamming AoEs and crowd control in the room because at that point, they're just buying time.