Best bet is to go with an affliction leveling build up until your late 50’s. The main one is called a drain tanking build:
https://classic.wowhead.com/talent-calc/warlock/3500250502201005--54500051000001/0BEaaaHKmfpffffSm2ACGbbbbhq
Your usual rotation will be to use Siphon Life, Corruption, and Drain Life on things. Life Tap when you need mana but try to do it between pulls. Start each fight with nearly full mana but down on health, maybe 50-80% health. Your Siphon Life and Drain Life will fill you back up, let your healer know you’ll be doing this so they don’t waste mana healing you up if they don’t have it to spare.
In instances I’d skip Siphon Life on trash, they will die too quickly to bother.
You should only Shadowbolt when you get instant ones through Nightfall. Both Corruption and Drain Life will give you a chance for instant Shadowbolts and you’ll get them pretty often. Drain Soul right before the mob dies if you need shards but only use rank 1, higher ranks are really a waste of mana.
It’s pretty much a waste of mana to use Curse of Agony on trash mobs and I wouldn’t use Immolate either if you spent all those points on improving your shadow damage. Get as much +spell damage and +shadow damage as you can, there’s a lot of lower-level crafted gear which will make your shadow spells very efficient. It will improve your Life Taps a ton and help you drain back that health.
Done properly you shouldn’t ever need to bandage or eat/drink as a warlock. Your mana and health will go back-and forth constantly and you can keep killing forever like that.
On bosses your rotation will be Siphon Life, Corruption, Shadowbolt, Curse of Shadow. Spam Shadowbolt and Lifetap/Drain Life when you need to. You want to end up at zero mana around the time the boss dies, then you can Lifetap to 50% health and mana and eat/drink up.
Raiding is a different matter entirely. Best build for that is, generally a DS/Ruin build.
https://classic.wowhead.com/talent-calc/warlock/25002-2050300142301-52500051020001
You’ll sacrifice a Succubus and your usual rotation will be Curse of Elements or Curse of Shadow, Corruption, spam Shadowbolt, Lifetap when necessary.
There are also deep Destruction builds that are worth it now that AQ is out but are a pain in MC and BWL since many mobs have fire resistance.