This is a three flag competition.
Each faction starts close to one flag of their own, and there's one in the middle.
The battle is very much about holding two flags, or rather rotating forces so that you always hold one flag more than the opponents. My experience is that you want to zerg the middle one as quick as possible while avoiding a humiliating loss of your 'starting' flag.
As a tank I've come to the conclusion that the best job I can do is to head for the enemy starting flag immediately. Not to take it, but to suicide. The reason for this is to tie up resources. In a best case scenario I'll have three or four enemy players spend well over half a minute killing me when I do my lonely assault. That is almost always enough for us to get the middle, or Water Works, flag.
After this the fight usually splits into one out of two scenarios. The enemy tries to hit WW while being careful not to lose their starting flag. I spend those fights rushing their flag alone, which is usually enough to see us winning by keeping two flags for the entire battle, and obviously never getting close to taking the third.
One useful trick is to get 'caught' by the enemy respawned players as they rush to hit WW again. As I play horde I also get the benefit of getting trapped in plain sight of those defending the Lighthouse, so they tend to stay there while I get chewed up by the force who ought to have let me be and hit WW instead.
The second standard battle is when the enemy hits our starting flag instead of getting caught up in a slug-fest for WW. Defending our flag is very often the most important job I can do now. The point isn't to kill the enemy players but rather to disable them from capping our flag by being a general PITA and refusing to die. If things start to get really dirty, a lonely rush for the enemy starting flag surprisingly often pays off. Just make sure noone else in your team isn't already trying to ninja-cap it before you head away, though.
It's also highly important to remember that this is a Random BG. You'll have players leaving their toons at the flag and going AFK in the hope noone will notice (players never leaving the spawn points are always noticed). Thus it's perfectly viable to hit the enemy starting flag in plain sight and cap it. You're likely to find out if there's really a player behind the keyboard of the defending toon.
I've used this defensively as well. As I can't see a stealthed player I'm forced into a guessing game. However, stealthed players tend to CC me before trying our flag, in which case I'll have to lay down AoE and notify my team. The best way to handle a plain-sight ninja attempt isn't to attack the enemy immediately though. Wait for them to start capping. Make sure you can break CC, and give them a slap. Preferably a ranged attack. Sometimes they'll try the flag again, and get slapped, and try again. Normally, though, you'll get caught up in a duel none of you can win, but that's good enough. The flag stays secure.
All in all this is a BG where you'll doing your job best if you keep dying slowly against multiple opponents killing you rather than doing tehir job -- capping or defending flags.