I had my character ready, black robes, black hood, and downloaded the Dawnguard DLC today.

So I start playing and a guard mentions the Dawnguard. Off I trot to this Fort Dawnguard looking to see how I can start serving my Master, Lord Harkon.

Alas I find it is the headquarters of the enemy! I ignore them as much as I can and refuse to join them (you would of thought them suspicious of me in my black robes, trying to look like a vampire?)

Anyway, they send me to this place way to the North East. I spend 30 mins trying to reach the cave which is high up a mountain.

I enter and overhear two vampires talking. I approach and am about to say "I want to help you fight against the wretched Dawnguard, take me to Lord Harkon so that I can help you", but they attack me!

So I turn around and run back out of the cave!

How do I reach Lord Harkon and join the Volkihar vampire clan?

I don't want to kill the vampires inside the cave, I want to join the vampires and help defeat the evil Dawnguard.

I am confused, surely you don't have to kill vampires before being able to join the Volkihar vampire clan? I don't want to kill vampires, I want to kill Dawnguard and serve Lord Harkon!

I want to serve Lord Harkon. They better not make me betray him.

Hate to break it to you but you got to kill those vampires in the cave, to advance the questline.

He's kind of a doosh.

Follow those http://www.uesp.net/wiki/Skyrim:Awakening.

Enjoy being a vampire.

Pretty much the same as everyone else said.

You have to "betray" (i.e. kill) the Vamps in the cave, free Serana and take her home. Other than a heavy conscience for all of thirty seconds, no ill comes from killing them.

But it's not really a loss. Consider it in a different light; you're proving Darwin's Law works in regards to Skyrim as well.

Even though its the spoilers section.. Gonna tag this anyway:

Harkon's chief bootlickers have sent Lokil and some grunts to search the tomb for something very important to their master. Lokil plans to use this to gain some influence within his master's court.

It is influence he does not deserve.

Show the fools how much stronger you are than they, and cut them down like they were thin-bloods. And then, when you bring Harkon what you've found in the crypt, he'll be elated at your prowess.

Of course, you'll still be a mere fledgling, only newly granted Harkon's blood. So you'll have to prove yourself.

The Volkihar place much emphasis on blood purity. The cretins in caves that call themselves vampires are in competition with you for cattle -- it's the survival of the fittest.

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That was fun to type.

Now, yes, you must kill or sneak around the hostile vampires in Dimhollow Crypt. Once in the court, you'll be killing other, "inferior" vampires.

Vampires place very little value on kinship, as you can see.

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You can more easily reach Dimhollow Crypt by traveling to Dawnstar, following the road until you reach the Hall of the Vigilants, and finding the path that leads up the mountainside there.

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Yes, you must speak to Isran to begin the questline, likely so that it's clear that you have options in whom you side with.

It is possible to go through the dialogue without saying that you want to kill vampires, FYI.

You'll get a chance to decimate them later. For now, consider it a bit of unpleasant, but necessary, fraternizing.