First thing I would say is that you want to be running a lot of instances because daggers are hard to come by if you are purely questing. Rogues are dependant on weapon damage. Backstab, ambush and sinister strike all deal weapon damage + extra. So if your weapon sucks then your damage sucks and you will have a bad time. Also try to fight one mob at a time. Use your thrown to pull and don't forget to refill the ammo. Combat spec is a lot more forgiving here.
I am not an expert but I found improved gouge helpful because it gives you extra time to get some energy back for a follow up backstab (Ambush/backstab > Gouge > Backstab > Eviscerate if it will kill or gouge again etc). It gives you some time to bandage too if needed. You can ranged pull into gouge for a backstab as well.
The you want to start putting points into Opportunity right after you finish Imp. Gouge so you have it maxed by the time you are 18. Unfortunately the next 5 points in Subtlety are a bit lackluster. But the rest of the tree makes up for it. I would put 5 points into camoflauge so it is easier to get behind your target without getting spotted if they move randomly towards you. I would grab Ghostly Strike next and then 3 points into Imp Ambush + 1 in initiative to open the next tier. Your goal here is to get hemo so 3 points in serrated blades and 2 more in initiative. Grab Hemo and preparation. Then I would grab 2 points in Dirty Deeds, 1 in master of deception, 5 in deadliness and finally premeditiation. Then I would finish in the assassination tree.
It ends up looking like this http://db.vanillagaming.org/?talent#fhxboxoZhZq0es0hRqo
The last point you can put wherever you wish. You are 60 by this time anyway so you can decide what you want to do from there.
Good luck.