Spellstone is probably the most underused warlock spell - especially as a new undergeared lock you should use it all the time. Just remember it clears ALL friendly buffs too, so don't use it right away with full buffs at the start of a battleground. Aside from the shard cost there is no downside to having an active-use offhand equipped and then switch in your passive with a keybind/macro after use or when you think you won't need it. Medicine pouch & skull of impending doom are the same: these are some of the best pvp items in the game, they're basically like a really good trinket that you can swap while in combat after using.
Succubus is okay for solo/ganking if you are horde, but still use felhunter for anything serious. Dispel magic is the best pvp ability in the game, and even though devour is a lot weaker version it's still amazing, esp on a pet that can dispel CC off you. Make sure to get a mod like detox, setup dispel priorities (i.e. devour sheeped priest/pallys first!), and bind it to something you can hit very quickly. Spell lock is only slightly weaker than improved counterspell, which is to say also amazing, and the blanket silence on it is really strong once you get a feel for using it proactively like when a druid shifts out to NS heal. Sometimes you can even devour the NS buff after silence if you get lucky! Paranoia is a nice bonus, esp if you have a hunter engineer in group (stacks with track hidden & catseye for +88 passive stealth detection). Felhunter is really survivable too, most people don't even bother trying to kill it as it can't be dotted up or two-shot with a scorch+fireblast like a succubus can.
Soul Link is good for 1v1/dueling esp if you are really undergeared but it lacks range/burst/utility for organized groups. Conflag builds are fun, esp 19/0/32 nightfall conflag is pretty viable imo, but I'd still recommend working on getting good with SM Ruin right from the start. Curse of exhaustion really changes the way you play in a lot of situations both 1v1 & group. 36 yard instant-cast 30% snare, lasting 12 seconds, with no cooldown, costing a negligible 100 mana, and can also be amped to 50% once every 3 minutes. It's also very hard to remove: priests & shaman can't do anything to it, pallys can only blessing of freedom it from one person on cooldown, druids are off hiding with the flag, and mages never decurse
Very strong ability.