So I have a warlock at level 36 atm, it is fairly twinked with mostly shadow wrath pieces (like 20g worth of gear, nothing incredible), my spec is full affliction until I max shadow mastery.
I fight mobs by opening with immolate - then depending on their level (= to me or higher gets CoA then corruption, lower just gets corruption) I rarely use siphon life unless they’re going to live the full 30s on it.
Make sure you have first aid as high as possible, it’s a lifesaver because you will be life tapping alot, you are very mana inefficient as a warlock and life tap is your primary source of mana generation.
Drain Life is relatively mana inefficient for a few reasons. If you get pushbacked on it at all, you are already losing mana efficiency to push you into net loss. You need either talents or shadow damage gear (ideally both) to push the damage and life gained per tick higher so that you can actually net profit off of it as far as life/mana goes. With some shadow damage gear and the 70% pushback resist talent in affliction this is fairly easy to do. I have rather high shadow damage at 36, about 140, but you’re netting profit as low as 50.
*Note: Mana efficiency is relative to the amount of LIFE:MANA conversion, if you aren’t at LEAST breaking 1:1 on life to mana you are losing mana in terms of life tap efficiency
For pets, I suggest succubus, she does magnitudes more damage than voidwalker, the CC she can do is amazing for adds/unexpected pulls and, if you’re on a server for it, she is a godsend in pvp (rogue jumps you? not on my watch). Even with NO shadow damage gear AND demo spec to buff him by 30%, with a full round of dots and just drain life, voidwalker won’t hold aggro, he can offtank something you have partial or no dots on but that’s it, his main use is honestly just to offtank something you can’t CC and to be sacrificed to give you an emergency bubble.
For gear, Shadow DMG is more important than everything else by a LARGE margin because not only does it increase your kill speed but it increases your return on life drain abilities AND the mana gained from life tap, reducing your downtime. Next up try to get a fairly decent balance of stam/int, every other stat is irrelevant while leveling.
If you can afford to twink yourself try to get some +stam enchants on your gear, +7 to bracers/boots and 50 health to chest are super cheap and make life alot easier, you’re gonna be low on stats if you’re using damage gear so it helps even it out. This setup has me able to comfortably fight 2-3 mobs at a time, completely murder people in pvp, and to take out elites of my level and below quite casually.
Always keep healthstone ready, if you have extra gold lying around, health potions of superior and below are dirt cheap (at least on my server) and I keep them around for emergencies.
Lastly a tip - don’t be afraid to fear, I can full dot something and fear it, and it’s going to die, you can also fear a mob off you defensively, and if they’re in danger of running into mobs, you can curse of recklessness them to break fear, same goes for mobs fleeing in fear when they get low on health, you can use this to manage mobs both solo and in dungeons.