First of all leveling as daggers can be painful even if you know the class completely but here's how I have done it.
First 7 points into combat tree
http://db.vanillagaming.org/?talent#fZGcGouge will be very important both for pvp and to get any chance to backstab in pve.
Imp sinister strike will be handy while leveling when you can't backstab.
30% crit on backstab is a must.
Next go sub into this
http://db.vanillagaming.org/?talent#fZGcZV0ecMore damage on "back" attacks, faster stealth movement, crit on ambush and extra cp on opener are all great ganker oriented talents.
Now spend some points into assassination like so
http://db.vanillagaming.org/?talent#fhxboxZGcZV0ecMore base crit, higher evisc damage, energy regen on finisher and extra critdmg on your already high critchance backstabs is all good.
Then move back to sub tree and work into this
http://db.vanillagaming.org/?talent#fhxboxZGcZAMecc0RYou might want to change imp sinister strike to dodge and consider dropping improved ambush at 60 in favor of master of deception since you'll open with cheap shot more than ambush.
But by then you'll probably have the hang of what talents suit you.
As for gear, you'll find the most good daggers on the auction house but since almost nobody levels with daggers they aren't too expensive. Look for a slow mainhand dagger. That is very important. Fast offhand is best but it doesn't matter all that much.
Go skinning/herb or mining for profs and make some money for daggers/gear.
If you find it too frustrating to get behind your target but love the sub tree then consider a hemo pvp spec. at lvl 33 or so that turns quite effective even for questing.
Hope this helps!
*Edit* As for playstyle, this spec will shine in the hands of a filthy unscrupulous ganker. Tactics are to wait for your opponent to be in a bad position to either defend themselves (having mob aggro) or simply not have enough health to survive an ambush/backstab/eviscerate combo. Once you get a feel of how much damage you can do consider giving your opponent a chance for a fair fight. :p
As for pve a nice opener is ambush/gouge/backstab and then fall back on sinister strike and whatever finisher you prefer. Improved sap is very useful for questing as well as pvp