Calcify- Will of the Sith- Chain Shock- Disintegration (at low levels, without sniper buff chances are you'll have around 10-15% crit chance, so against one foe you'll get an affliction crit every 18 seconds or so- which makes the 1% heal truly not worth it from parasitism. Once you're around 20-25% crit and have wrath, crits are going to be more common, for now parasitism isn't worth it)
-Madness- Death Field- Sith Efficacy- Wrath

At this point- you can take Parasitism* if you want to, or you can simply jump into another tree- personally, I'd go

Convection- Reserves- Electric Induction*- Exsanguinate*- Lightning Barrage

At this point it depends. If you're going to pvp alot- Electric Bindings- Chain Lightning- rather than Elect Induction/Exsanguin take Lightning Barrier and Subversion- then Backlash and Lightning Storm

If you're just soloing- Dark Mending is nice so you can heal when needed- Seeping Darkness for crit- Force Suffusion for 10% more DF damage.

Or- keep going up madness, Deathmark- Lingering Nightmare- Force Horrors- Creeping Death- Devour- Creeping Terror.


Ultimately- as long as you don't go corruption you'll do fine solo... though, if you go corruption, you'll have better chance with getting groups.


*- I put a star beside abilities that aren't particularly important and in general are just filler to get another ability.